Tuesday, November 21, 2006

AV - The Air Brush Trick

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It's not much of a trick. But then, it's not much of an air brush. $4.99 from Harbor Freight. #6131. Little jobbie made in Taiwan, but not very well. Made, that is. Oh, it'll blow paint. Any airbrush will blow paint.

Air brush is a syphon-type spray gun. The paint is in a little screw-top jar that plugs into the gun. The gun is about the size of a ball-point pen. The jar has a hole in the lid so the thing is open to the atmosphere. The jar hangs under the nozzle of the gun. To make it blow you press a little button on the gun. Air blows across a tiny orifice positioned in the throat of a venturi. The orifice is actually a needle valve but we'll get to that in a minute. A suck-up tube runs from the orifice down into the jar of paint. When Mr. Bernoulli reduces the pressure on the venturi-end of the suck-up tube, atmospheric pressure pushes the paint up the tube and out the orifice, where the stream of air blowing past atomizes the paint and blows it out the end of the gun. So mebbe we should call it a push-up tube.

There's not much in the way of control. The paint comes out in a circular pattern, about the same as you'd get from a spray can. And while you can't control the shape of the pattern, you can control the size of the pattern. You can also control how much paint you deliver by adjusting the air pressure. Plus, you can thin down a thick paint, lay it on in thinner coats. Or leave it thick. Air brush don't care. Thick stuff just takes more air pressure. And comes out in bigger particles. Air brush will spray molasses if that's what you want to do.

So there you are with this itty-bitty spray gun. Jar only holds two ounces. If you're an artist, you buy a lot of jars, fill them with different colors of paint; Keep on Trucking and all that sort of thing. To spray a different color you unscrew one, screw on another, after flushing the suck-up tube in a jar of thinner in between. Just open up the needle valve to flush it out good.

Of course, two ounces isn't a very useful amount, not if you're building airplanes. But just as artists use different bottles, so can you. Larger ones.

I use gravy jars. Heinz. Usta have 57 Varieties. Gravy jar holds about six ounces. I tried Spanish olive jars for a while but they didn't have the right wrist action. Some buys use baby food jars but I've become a confirmed Gravy Jar Man. You can't use a really fat jar because the air hose screws on to the bottom of the gun and gets in the way. But you can use a tall skinny jar, which is what I did during my Spanish Olive period.

MAKING A BIGGER JAR

Got a 7mm deep-socket? Take the air brush jar apart. No, not the gun, the JAR. You'll have to pull the suck-up tube off the fitting. If it's stuck, heat it. Don't damage it, you'll need it later. Once the tube is out of the way use your 7mm deep-socket to loosen the nut holding the suction tube fitting to the lid. Take it apart and keep track of the washers.

Got a Unibit? A Unibit makes drilling holes in thin stock a breeze. Your new lid is thin stock. Drill a pilot hole for the Unibit then open it up to accept the air brush fitting. It's about 6mm in diameter, which is close to 1/4" so try that.

While you're drilling, put a #60 hole in the lid out near one edge. That's your vent, the thing that lets those fourteen point seven pee-ess-eyes push the paint up the suck-up tube.

Now put your new lid together. Transfer the hardware from the air brush lid to your Gravy Jar lid. Make sure the vent hole is at right angles to the outlet of the suck-up tube. If you don't, you'll end up spilling paint all over yourself. (At 90 degrees from the axis of the spray gun, the vent hole will always be near the apex of any tilt you put on the bottle. If you position the hole anywhere else, there will be some tilt-angles you can't use.)

Your bigger jar will need a longer suck-up tube so go punch a hole in a used spray paint can. Let the residual propellant escape then cut the bottom off the can. Inside you'll find a free marble (!) and a length of suck-up hose about 7" long. That's too long for your Gravy Jar but you can cut it down. Except it's also too big around - it won't fit on the suction fitting in the lid. Metric vs whatever and all that. So go find the original air brush suction tube. Snip off about half an inch and slide it onto the suction fitting. Now slide the spray can tube over the smaller air brush tube. It won't want to go but it will, if you heat the larger tube and put a little muscle into it when you slide it on.

Why the tube out of a spray can? Because you need tubing that will withstand MEK and toluene and lacquer thinner and whatever else you might use. You can buy such tubing but the minimum quantity is about five feet and it costs the earth. And there you are, with half a dozen useful lengths of the stuff inside old spray cans. Plus, you get a free marble.

So how long does it have to be? It should reach the bottom of your Gravy Jar. Not smack up against it, but pretty near. You'll also need several extra jars plus one extra lid. (Why an extra lid? Because you've just converted one of them to your sprayer-lid.)

That takes care of making your new lid, which is 90% of the job. But like everything in homebuilding, the remaining 10% of the job will take 90% of the time.

MAKING IT USEFUL

See that crappy little black plastic air hose that came with the air brush kit? It's going to pull off its fittings about ten minutes after you start using the thing. So go find some fine stainless steel safety wire, put a couple of warps around the fittings and twist them tight. Po' Boy hose clamp. Works, too. Snip off the twist and tuck the end flat. A bit of tape over the safety wire will keep you from cutting yourself. (And yes, that does happen to be a Band-Aid on my airbrush hose. No, I don't want to talk about it.)

The air brush kit comes with a fitting that connects to cans of compressed air (!) Seriously. They sell canned air at hobby shops and the like. They don't hold much air but if you're painting toy trains or model airplanes, you don't need much. For painting real airplanes, on the other hand, you need a buncha air. Mebbe two bunches, before you get it all done.

To provide air to the spray gun you need to buy an adapter that will mate with the fitting on the hose in the kit. Harbor Freight sells one, #P-1655. It's got a male thread on one end to match the fittings on the air brush, and a female 1/4" NPT on the other. To make it match my air line I added a quick connect. Harbor Freight sells those too but I don't know the number.

The 1/4" airline adapter (P-1655) allows you to use your air compressor. Or any other source of compressed air that has a 1/4" NPT fitting hanging off the end. Such as a big tank of air for refilling a tire. Which will blow more paint than one of those itty bitty cans from the hobby shop.

The air brush is sensitive to air pressure. If using canned air, the lid has an adjuster built in. But when using an air compressor, air tank or whatever, you'll need to get yourself some form of pressure control in the line. Most of the time you'll only need five or ten pounds but with thick stuff you will need as much as 90 psi. Because the gun is easy to adjust you can get by with a very simple restriction-type ‘regulator.' (It doesn't do much regulating but it does cut down the pressure.)

IS THIS REALLY PRACTICAL?

That depends on your financial situation. Sixteen ounce rattle can of ZC primer only has about seven ounces of paint. That makes it pretty expensive. Plus, the nozzle is always clogging up and every time you want to use yellow all you got is green or visa versa.

Quart of primer contains a quart of primer. Cut it 1:1 with reducer, you got two quarts. That's sixty-four fluid ounces and that works out to about one eighth the cost of using rattle cans.

And then there is other stuff to paint, such as steel parts, which get a baked coat of primer then a baked coat of gloss black or whatever. Having a little spray gun that's easy to set up and simple to clean makes blowing a little paint quick and inexpensive. It takes about a minute per rib to give them a coat of ZC with the air brush.

SPARE PARTS

At five bucks a copy you can be pretty sure your airbrush won't outlast the Pyramids. And it is going to accumulate some wear. It's got a couple of teenie tiny O-rings that probably won't cost more than ten dollars each, plus the hernia you'll get from digging through the McMaster-Carr catalog trying to find them.

So buy two kits. The second is to provide spares for the first. At five bucks a pop it's the logical way to go.

USE, STORAGE & CLEANING

Get yourself a box about a foot on a side. Cardboard is okay. Give it a coat of paint; mebbe stiffen it up with some stringers. Use it to store your Air Brush Kit. Keep the thinner and spare jars and spare parts and stuff like that in the box, along with your pipe cleaners.

One of your jars should be filled with thinner. Lacquer thinner will do to clean up after most primers. The other jars are filled with whatever you want to spray, from kerosene to zinc chromate. To give the glass jars a bit of protection, wrap them with several turns of old fashioned friction tape – the rubberized cloth stuff.

Store your pre-mixed primer, tightly sealed, in the refrigerator. If you've got kids in the house, store the stuff in an ammo can out in the shop. Keep it on the concrete deck, covered by several layers of cardboard to keep the stuff cool.

Every time you use your spray gun you gotta clean it. Takes mebbe five minutes, tops. Here's how.

Unscrew the paint jar, put the lid on it and put it back in the refrigerator. Plug your sprayer lid into your jar of thinner and blow thinner into a rag or can until the suck-up tube and nozzle are clear. Get a little thinner on a rag and find your pipe cleaners. Dip a pipe cleaner in thinner and have it ready.

Shut off the air, disconnect the hose and put it away in the storage box.

Take a look at the gun. See that little circlip? Open up the screwdriver blade on your jackknife and pry off the circlip. Now you can push the brass needle valve down through the body of the gun. Unscrew the needle valve as you go. The cone of the valve stays on this side, only the needle part pushes down & out of the gun. Unscrew the nozzle.

That's it. Three parts. Four, if you include the circlip.

Use your pipe cleaner to ream out the needle valve and the nozzle. Get ALL the paint outta there. Use the rag to wipe any paint off the suck-up tube and sprayer lid.

Once everything is clean, put it back together and store it in the box.

MAKING A SPRAY BOOTH

The last part of your spray-paint kit is the spray booth, which looks suspiciously like a cardboard box. Hang it or nail it near to a window and rig some dryer ducting from the box to a panel in the window – you want those fumes to end up in the neighbor's air conditioner intake, not in your shop. Rig a small fan to pump air out of the booth and into the ducting. In case you hadn't noticed, a fan is an air pump. To make it work like a pump wire & tape it to the wall of your spray booth. On the outlet side, rig a plenum chamber (which also looks suspiciously like a cardboard box) and fasten your ducting to the plenum chamber.

Making a spray booth is one of the few times in your life when you can use duct tape on a duct. Enjoy.

You don't need a very big fan because you aren't using a very big spray gun. The boxer fan out of a computer power supply will work but most of them run on 12vdc and are noisy. I use a cheap (cheap!!) import desk fan, about six inches in diameter.

To make cardboard useful it needs to be stiffened with wooden stringers. Lath will do fine. Use urethane glue and staples. Once the glue cures, give the box a coat of white house paint and repaint the interior periodically, not only to renew the white surface but to bind up any over-spray residue. After you finish your plane, cut the box up and get rid of it.

When adding stiffeners to your spray booth, be sure add a couple to the inside, up near the top of the box. Rig a couple of wires up there. To paint stuff you'll typically hang it form hooks of bailing wire and use a wand of the same stuff to hold the part in position while you shoot it with paint. Most primers dry in a matter of minutes but it's a good idea to rig a curtain on the front of the box so you can leave parts hang while they dry. In a cold or damp climate, adding a door and a couple of light bulbs NEAR THE BOTTOM will turn your spray booth into a drying booth.

The spray booth is to keep from blowing toxic residue all over your shop. Everyone worries about hexivalent chromium but the truth is, alkyd and epoxy resins, when in aerosols, are just as bad for you, although for different reasons. So even though you have a spray booth that passes your problems on to the neighbors, any time you blow paint you should dress for the occasion. That means an air mask or industrial quality respirator, and gloves that are impermeable to the thinner & vehicle in whatever paint you're spraying. (And this applies to rattle cans as well.)

Most volatile vapors are explosive. Try not to weld while painting, nor to paint while welding. And the smoking lamp should be out.

PASSING YOUR PROBLEMS ALONG

Back when Tony Bingelis was painting his Falco one of his neighbors complained about the smell. I told him he didn't have a big enough cat box.

The average cat box is 20" long by 14" wide. So you build a box 20-1/2" long by 14-1/2" wide by about 30" tall. The box slides down over the cat box, which is four inches deep and contains 2" of water. Inside the box you've built, there are five partitions spaced about 3" part, except for the input and output which are about 4". Two of the partitions are open at the top but extend down INTO the water. The other three partitions are sealed at the top but only extend downward to within one inch of the water.

Any place you need to form an air seal, such as along the edges of the plastic cat box, cut up some urethane foam, give the surface a spritz of upholstery glue and stick it to it.

On the output of your cat box you put a large fan positioned to suck air through the box. The inlet is on the top of the box at the far end.

What you've just built is a kinetic air filter. Each time the air is forced to reverse direction, which it must do to negotiate the partitions, heavier particles will tend to collide with the surface of the water and stick there.

Even a one cat box unit is surprisingly effective at eliminating odors and trapping paint particles. But if one unit isn't enough, add another to either end. When combined with a labyrinth filter at the input (ie, a stack of furnace filters) and an electrostatic particle precipitator on the output, you can get clean room conditions with only seven reversals (ie, a two cat-box unit).

If you rig such a unit for your spray booth the odds are your neighbors will never know when you decide to blow a little paint, since none of it – nor any of its vapors – will get out of your cat box.

The Cat Box Principle also applies when painting the entire airplane. Rig an air dam across the garage so the door comes down onto the dam and install the cat box in the middle. Seal up any gaps with cardboard, foam and duct tape. To get a flow of air into the shop, use a window or door on the opposite side of the shop. Build a light frame of wood to fill the doorway or window. Glue furnace filters to the frame. Put the frame in place any time you want to blow paint. It won't stop a sand storm but it'll keep out the bugs and most dust.

A WHIFF OF REALITY

So you go down to the local EAA chapter and tell the folks about your nifty little air brush and your cardboard spray booth and your cat box air filter... and everyone will tell you what a bad idea it is. Spray cans are better. Zinc Chromate is evil incarnate. And if you use 6061 and pop rivets you don't need corrosion protection, or that noisy air compressor or all that other stuff. Besides, if the designer wanted the part protected, it would have arrived that way IN THE KIT.

About there I realize I've stumbled in to a meeting of the Dunkin Donut Kit Assembler's Association and slink back to the shop to continue making curvy bits out of flat bits. And giving the curvy bits a spritz of ZC when I get them done, because, not having a zillion dollars for a kit (or even $10,000), I've no idea how long the part will sit on the shelf before it can be assembled. That spritz of ZC will keep it from going bad. What I do know is that building a Teenie Two will cost me about $500 and a CH-701 about $1,000 (less the engine in each case), because I'm only paying about two bucks a pound for the materials, right down to the solid aircraft rivets, gleaned from various surplus sources at scrap metal prices.

Which is why a $5 spray gun and cardboard spray booth and cat box air filter make good sense to me. An' besides, I get a free marble.

-R.S.Hoover

AV - Cutting Stringers

In a related thread (Cutting aluminum etc) 'Jim in NC' reminded us of the need for an auxiliary shoe when cutting thin stock on a table saw.

This also applies to cutting stringer stock.

Although you may order spruce stringers cut to size, if you buy a spruce 'kit' the stringer stock will usually arrive as a number of six-inch planks finished to the required thickness. You are expected to set up your table saw to the second dimension and rip the planks.

The standard shoe (ie, the removable panel through which the saw blade extends) allows too much clearance for the cutting of 1/4" and 5/16" stringer stock on a production basis. You must either make up a new shoe or use an auxiliary shoe in the manner described by Jim. I think the use of an auxiliary shoe is the more common procedure since the cutting of stringer stock also dictates the need for finger-boards and the like. (Using an auxiliary shoe allows them to be fastened to the shoe.)

As with the sawing of aluminum, ripping stringer stock is a procedure common to the business of building airplanes. Unfortunately, the success of many such tasks depends on a host of details largely unknown to the novice yet seldom mentioned in the literature. Experienced builders tend to forget that a novice must be told everything, such as the need to dress for the occasion when sawing aluminum... or that a jig is needed when cutting a scarf joint, a zero-clearance shoe when cutting stringers and so on.

Jim's message caused me to recall a comment by a local builder about having to buy finished stringer stock because he was unable to accurately rip a quarter-inch plank into quarter-inch square sticks for his ribs. The builder was an experienced wood worker but looking back on it, I suspect he didn't realize he'd need an auxiliary shoe for cutting such small stock.

-R.S.Hoover

AV - The Skin Game

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Working out back of the shop under the shed roof. I'm cold and in a lot of pain. Khaki's, flannel shirt & a shop apron was warm enough when I started, afternoon sun heating up the tin roof. Not too bad at all. Except for the pain, which I pretend not to notice. Then the sun did it's trick and the breeze picked up. Eight to ten miles per hour according to the homemade anemometer spinning busily above the shop. But it's been known to lie.

California cold isn't really. Nothing at all like Duluth cold or Fairbanks cold. But I've been bent over the bench about four hours now and the pain is becoming The Pain. Kidney stone; something new for me. (If you haven't been there, you won't even come close :-)

A jacket helps. As does a cuppa coffee. But when I go back out to the shop the wind has picked up and spates of rain are coming in from the coast, invisible now, all light gone from a sky filled with dark scudding clouds. But no pills. Not yet. Not until I finish deburring the holes I've spent all afternoon laying-out and drilling.

Wing skins. Not big; two by eight feet, plus some trim. Outboard wing panels, upper surface. Only nine ribs. No stringers. But a lotta holes.

The holes for the spars reside in a couple of pieces of eighth-inch by one inch drug-store aluminum, the holes for the ribs are in a yard stick hijacked as a template. I used the computer to lay-out the holes, pricked the bullets, center punched the prick marks, drilled them on the drill press with the bit spinning 3100 rpm and the swarf winding away in a spiral as bright as a diamond. And then deburred.

The skins, left and right, stuck together belly to belly with a few squirts of spray glue, are laying on an old door pieced out with some scrap. The trim allows a couple of free holes to accept the cleco's that fasten the skins to the table.

Layout is school-boy geometry. Stretched black thread for my X, carpenter's square for my Y, corners trammeled with a twelve-foot piece of extruded angle, the intersections carefully marked with a Sharpie. I can live with plus or minus sixty but smile when the marks cross each other with an error of perhaps fifteen thou in 98.95 inches. Close enough.

The ribs have a riveting flange five-eighths of an inch wide with the Safe Zone being the middle third. For a safe structure the AN3 flush-heads should end up somewhere within a square about three-eighths inch on a side. But closer is better which is why I use templates for laying them out. Centered on the line then squared, the holes fall almost perfectly within a square only an eighth of an inch on a side. This is good. This is why I've spent four hours bent over the bench with the wind blowing up my ass. (Should of put on the jacket sooner. Should have bought Xerox in 1957... )

Once located relative to the line of holes for the spar cap rivets, the rib template is cleco'd to the panel and the twenty-six holes are drilled. The swarf is brushed away, the template moves to the next rib, the location is verified (BIG red arrows, hard to miss... but not impossible) and you do it all over again. Not a big deal. But the culmination of prior work spanning months.

After drilling comes deburring. Then flipping the panels over and deburring again. Dog-leg deburring tool, finger as a gauge. The skins are sixteen thou. Gotta be careful with the deburring, keep the skins supported. They'll be dimpled later and it's tough to get a burr out of the bottom of a dimple.

The only secret to building airplanes is to do something every day. Doesn't matter what you do or how long you do it, what matters is the work-habit. Do something every single day and you'll be surprised how quickly you run out of things to do. When that happens the only thing left is to climb in and go for a ride.

Today it was some holes in the wing skins. Tomorrow it will be something else but today is past and the holes are done and I get to come in the house and gobble a handful of pills and feel The Pain crawl back in its cave.

It's raining now but the coffee is hot and the pills have kicked in. Just another day in the solitary art of Flying Machine construction.

-R.S.Hoover

AV - Making Boxes & Things

Guy come by the shop last weekend. Wanted me to make him the slider dingus that goes inside the short strut on a Cuby landing gear. No plans, just a badly done sketch.

"What's it made out of?" I ast him. He didn't know, guessed it was steel. "What kind of steel?" I ast him. Blank stare.

I went out to the little shed, chased the possoms off the top shelf, found the CUBY file box, pulled it out. He'd followed me, stood at the door acting like he's never seen a possom before. Or a stack of blueprint filing cabinets. The Cuby plans weren't in the cabinet. Like most plans they have their own box. He helped me put the V77 box back on the shelf then followed me back to the shop.

Dingus was shown as steel, no alloy specified. It's not a very good drawing. (Drawing reference about 18-D) I flipped through the plans looking for a more detailed drawing but don't find one. Which doesn't mean it ain't there.

The dingus is properly called the ‘slide' and acts as the carrier for the AN4 bolt that is the travel-limiter on the Cub's gear. It goes inside the short strut, which is 3/4 x .058 so the OD of the dingus is called out as five-eighths, which should work. Drawing shows it as being just half an inch thick with a significant chamfer, which won't work, unless you've got some dimensions to go by. I point out the missing dimensions which makes the existing dimensions questionable. He waves a lot of money at me and I am wooed into making him a couple of dinguses (denguii?).

The guy quickly lost interest in watching me set up the lathe, started nosing around the shop, which I don't like. People steal things. Even nice people like fellow pilots, or rich people like most homebuilders. (Yeah, I know. Richer than me, though.) I sat him down in the patio and gave him the set of plans to play with while I made his dinguses.

I didn't have any five-eights rod but I had a stub of free-machining three-quarter stock so I used that, turned it down to .625. I made the dingus five-eights in length with a sixty-thou chamfer on each end. Drawing said the quarter inch hole was drilled, which would probably work in most cases, so I laid them out, set them up in a vise on the drill press, piloted them to something smaller then opened them up with a freshly sharpened quarter-inch bit. Broke the edges, buffed them up, tried them with an AN4... kinda tight but good enough for a Cuby.

He admired the shiny little slugs of steel then tucked them in his pocket and asked, "Did you make this?" flapping the lid of the plans-box back & forth like a punkah on the ceiling of a bungalow in Krishnapur.

A plans box is just a shallow plywood tray with a hinged plywood lid. The hinge on this one was a piece of leather salvaged from an old pair of boots. Of course you make them, for crysakes. I just looked at him. Okay, maybe I rolled my eyes or something.

"I've never seen anything like it," he said. Which told me he hadn't been on many construction sites. Or boatyards. Nor built many airplanes. After he left I put some moth crystals in the plans box and put it back in the shed and went back to what I was doing, which has banging a bulkhead out of some .020.

Plans usta be blueprints - blupes. Blupes fade so you store them in the dark. Most modern drawings don't fade but over the course of time needed to finish most homebuilts they tend to get torn, damaged, dirty or lost... unless you do something about it.

Real shops, you've got a filing cabinet for your drawings. And that is one humongous cabinet. When I worked for Vernon Payne he had three of the things, about four by six feet on a side and three feet high with about a dozen drawers in each. Building just one airplane, and working from just one set of drawings, there's no way to justify the cost or space for a flat file cabinet. So you build a flat file BOX. Small box, you can hinge the lid. Big box, as in three by four feet, the lid usually attaches with latches.

What you build it out of depends on you and the night and music. Building a metal airplane? They you'd probably use aluminum. I've made one out of fiberglas but I was just showing off. Mostly, I make them out of wood.

Everybody does this. Don't they?

HOW TO MAKE A PLYWOOD PLANS-BOX

Start with a frame of one-by-two pine. Make the interior about one inch larger than your drawings in each dimension. Assemble the frame using glue and dowels or corner blocking. Don't use any nails; we're going to run it thru the saw later on.

Skin the box with 1/8" luan plywood. If you use staples or brads, pull them out after the glue cures.

Clean up the edges then run the box through the saw ON EDGE so as to cut the one-by-two. As soon as you cut one side, tack some plywood over the cut. Cut all four sides, being sure to keep the same face to the fence so the cut will come out even all around.

Clean up the cut line, clamp the box back together and install a piano hinge opposite which ever side you want it to open. Give it a seal coat of 50-50 varnish:thinner, let that cure for a couple of days then give it a light sanding, wipe down with thinner and lay on a full coat of varnish. If you remembered to varnish under the hinge the thing will be good for about fifty years, give or take a decade or so.

That's the DeLux model. I've got a couple like that but most are more along the lines of a Jeep than a Cadillac. The frame is often pieced out from scraps of 3/4" square stock or whatever happened to be available when I needed to make a plans box. Some use cardboard for the skins instead of plywood, which works okay if you add a few stiffeners and give the cardboard a couple of coats of oil-based paint.

It helps if you start with a frame that is square so use corner clamps if you've got them. But close enough is good enough in this case. Lots of times I use scrap to make up a pair of open trays rather than a box. When I get around to it I clamp them together, true up their edges by running them through the bandsaw then do a few passes against the disk sander. That gives the box a uniform outside appearance and a nice neat edge. Inside, the thing may be crooked as hell but the plans don't know the difference.

The types of hinges I've used range from real hinges, including some aircraft stuff, to anything that has enough flexibility, such as leather or upholstery material. Fabric or leather hinge, you can glue it on if you remember to cover the parting line with tape so as not to glue the box together. If you use brads or screws make sure you have enough depth of wood to provide the support required. Flat belting used to be the first choice for this sort of hinge but you don't see it around much any more.

If you want to make it air tight, run a bead of RTV along the closing line, cover it with waxed paper and leave it closed until the RTV has cured. Then clean things up with a razor. If it's a DeLux version, such as a gun case or whatever, you can route the edge to accept a strand of O-ring stock.

The latch is usually a couple of round head brass brads and a piece of safety wire but anything that does the job is acceptable.

WORKING WITH WOOD

Discovering that every aircraft mechanic needs to be a pretty good woodworker always comes as a bit of a shock to guys wanting to build an airplane out of plastic or metal or steel tubing. If you want to build an airplane - ANY AIRPLANE - and have no experience working with wood, it might be a good idea to get some. Making a box for your plans isn't a bad way to start.

Nowadays, spend a thousand dollars for a good tool, it's liable to arrive in plastic bag. Making small boxes of every conceivable size, shape and strength is a recurring chore for anyone who has a few machine tools because if you just throw stuff in a drawer the odds are it won't be usable when you need it.

THE BEAUTY OF BOXES

Airplanes aren't very heavy. (You wish!) Whole wing's-worth of ribs, metal or wood, only weighs a couple pounds. But bulky and fragile pounds.

Odds are, someone is building the same airframe as you. A good building strategy is to become a part of a group working on the same design. If you can come up with a safe, inexpensive method of shipping stuff back & forth, having one guy do the wing ribs while another does the tail and a third does this and the fourth does that... often saves a lot of time in producing the components.

Parcel Post offers dirt cheap shipping for a container of a given size and weight. Not fast, just cheap. A sturdy plywood shipping container made of pine stringers and eighth-inch plywood is strong enough to protect fragile components yet light enough so that most of your money goes for shipping the contents rather than the box. And of course, the box is reusable. At one time this sort of thing was pretty common among homebuilders. Me and a few other fools still do it but the very idea of helping each other as we build our thousand dollar airplanes has fallen below the dollar-oriented radar of today's homebuilt community.

The ‘Experimental - Amateur Built' licensing category exists to promote EDUCATION. If you're willing to jump through U.S. Postal Service hoops you can even ship your parts back & forth at a special rate for Educational Materials.

Man really CAN fly... with a little help from his friends.

-R.S.Hoover

AV - How to Make Ribs Out of Old Orange Crates

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A fellow who has been following my posts about building airplanes on the cheap wrote to say he'd found plenty of straight grained hemlock 2x4's at his local Home Depot. Then he said he couldn't use it because it was full of knots. I smiled at that, fired off a message telling him not to worry about the knots. The knots would vanish as the work progressed. That got me a rather grumpy reply about wasting his time.

Apparently the fellow doesn't believe in magic :-)

Magic has been defined as anything we don't understand. At one time the ability to do long division or work with fractions was considered magical. Making knots vanish is no more difficult than long division.

STICK RIBS

Have you ever made one? Probably not. Which is kinda sad because stick ribs are a key factor in building a strong, light-weight, efficient and inexpensive wing. Once you know how to build a good wing the rest of the airframe is relatively easy.

To give you some idea as to what I'm talking about I've provided a couple of photos of stick rib construction and a drawing of the airfoil. Mr. Ryan Young, the Coast Guard's answer to Samuel Pepeys, has allowed me to pin the illustrations on his electronic wall.

(See: http://users.lmi.net/~ryoung/Sonerai/How_to_Build_Wing_Ribs.htm )

Making ribs is a fundamental Rite of Passage for homebuilders, each of whom has their preferred method. Some argue for fitting each individual stick, others for leaving them square; some insist on clamps versus fasteners, other feel the only practical fastener is a Monel staple; some use circular gussets, others are polyform but most use some version of a triangle. And then comes the Glue Wars...

Oddly enough, the myriad differences in rib construction vanish along with the knots as soon as you get some daylight under the wheels. All of that tiresome but socially-required Homebuilder Bullshit boils down to the basic questions of strength, weight, cost and convenience. Build your ribs in whatever way best matches your situation. So long as they are light enough and strong enough, the airplane will fly just fine. (As a basis for comparison the rib shown in the photos takes about fifteen minutes to assemble using 1/4" aircraft nails, weighs about 3.5 ounces and costs about fifty cents, all tolled.)

MAKING STICK RIB STOCK & VANISHING KNOT HOLES*

The longest pieces of wood in a stick rib are the cap strips - the top and bottom pieces. For a big, old fashioned airfoil the cap strips are liable to be five feet long. The other sticks in the rib, the verticals and diagonals, are shorter; maybe a foot long, max, with the others being even shorter than that. On a modern airfoil, say a 4415 having a chord of 48" and built with a D-cell leading edge, the cap strips are about three feet long and the longest diagonal is maybe nine inches, something like that.

The above is worth mentioning because it will help you understand why the knots drop out of the equation.

To turn a two-by-four into rib stock you set up the saw for whatever thickness you want then rip the two-by into laths. Ignore the knots, just cut right through them. If the lath breaks at a knot, don't worry about it. Once all the two-by-fours have been ripped into laths, run them back through the saw to produce rib stock. If you want square stock you don't even have to reset the saw.

When using locally available lumber the wiser course is to give yourself plenty of extra wood to work with. I generally cut about twice as much as needed, selecting the very best of the bunch for ribs.

When ripping the laths into rib stock you will again cut right through the knots. And again, some of the pieces will break at that point. No big deal.

After you've reduced your two-by-fours to rib stock you'll have a huge pile of sticks. Some will be full length, others will be shorter, having broken at a knot. Don't worry about it. We'll use the long pieces for our long sticks, short pieces for short sticks, discarding any knots we encounter along the way.

Now comes the grading. You're looking for clear, knot-free pieces having flat grain with a run-out of at least one-in-fifteen and long enough to serve as cap strips. Put the pieces on your work bench and examine the grain. Looking down at the piece, the grain should be looking up at you. If you can't see it, roll the piece over to expose the other edge. If the grain is not clearly visible on one edge or the other, discard the piece.

It isn't uncommon to find grain of sixteen to twenty in a two-by-four of Western Hemlock. The more gain, the higher the density and the heavier the piece so don't go overboard here. I'm happy with two to four annual rings across a quarter-inch rib stick.

Although grain run-out is a critical factor in the strength of the wood, any run-out greater than the length of that particular piece is acceptable. For example, on a diagonal that is nine and a half inches long you should be able to follow the grain for the length of the piece. That means almost any stick will meet the specs for the shorter members in a rib. The reason why this is so has to do with the manner in which wood carries a load and may be confirmed by a few simple experiments.

Once you've selected the pick of the litter for your cap strips, you're home free; the rest of the job is a no-brainer. Cut your cap strips to length and bundle them using rubber bands. Use the residue to make up the vertical and diagonal pieces.

WORKING WITH WOOD

I haven't bothered to mention the need for finger-boards (some call them ‘feather-boards'), pusher-sticks, hold-downs and a zero-clearance shoe. I consider this sort of thing to come under basic woodworking skills and have assumed anyone building a wooden wing or fuselage would be familiar with them.

Fabricating ribs, stringers, longerons and spar caps from locally available materials assumes the as-sawn rib stock will have a reasonably smooth surface and be of the required size. If you don't have a good saw or if you aren't a good sawyer, it might be wise to enlist the aid of someone with better equipment or more experience.

Some folks, mostly those who have never built anything, will argue that all of the wood in an airplane should be milled to size and free of splices. Wood that has been planed smooth certainly presents a more attractive appearance and I strongly recommend it for furniture. But as-sawn stock works perfectly well for ribs. In fact, if you order rib stock from a spruce supplier it will arrive in planks of the required thickness (i.e., one-quarter, five-sixteenths, etc) which you are then expected to rip to the required width.

Of course, if you're a skilled woodworker with a shop full of tools that includes a surface planer (not a joiner), and if you can afford the additional wastage, then you would probably mill all the stock you need to the required size. But many simply can not afford either the tools nor the wastage. Accurately sawn, with an eighth-inch kerf, about 60% of the material will end up on the floor as sawdust. Even allowing for the smallest possible clean-up cut in the surface planer will raise the wastage to about 75%

As for splicing, a scarf of twelve-to-one will typically equal the strength of the parent material and is often stronger. Just as we must splice short pieces of plywood in order to make long ones, when using locally available materials for our spars, caps and longerons, splicing is usually required not only to make up the required length but to eliminate any knots. All of the handbooks on the maintenance of wooden aircraft components contain examples of proper splicing procedures.

-R.S.Hoover

*Used in the theatrical sense, where to vanish something means to make it disappear.

VW - Polished Crankcase II

Polished Crankcase

Don’t do it. Not if you intend to drive the vehicle. If it’s something for display, feel free to polish it to a nice shine. A coat of clear laquer will preserve the polish for about a year.

But if you intend to drive the vehicle, give the cleaned, deburred crankcase one final wash with hot soapy water followed by a boiling water rinse and allow it to air-dry. A touch of compressed air through the oil passages would be wise (and I assume all plugs are out).

Your clean, dry case should be protected with a thin coat of flat black paint on its exterior surfaces. Do not use a hi-temp paint as the high clay or eutectic metallic content that gives such paints their high-temperature qualities acts as a thermal insulator. What you want is a surface that will radiate heat. Polished surfaces reflect heat. If you polish your crankcase it will run considerably hotter than normal.

This isn’t an automotive hints & kinks sort of thing, it’s simple physics. Veedub drivers in cold climates have long known the benefit of chrome plated valve covers and push-rod tubes. The heat-reflective surfaces cause the engine to run from ten to thirty degrees hotter.

The original Volkswagen engines (1935-37) was designed for a service life of 100,000 km; it didn’t even have replaceable bearing shells. But through the use of full-flow oil filtration systems the service life of a properly assembled VW engine can exceed 300,000 km, which means the engine may be exposed to the elements for 20 years or more, and that justifies a protective coat of paint.

Flat black paint is virtually transparent to heat radiation. Giving your crankcase, push-rod tubes and valve covers a coat of flat black paint atop bare metal actually promotes engine cooling. One of the quickest ways to spot a professionally built engine is from its somber flat-and matte-black surfaces.

VW - Pulling Dowels

Pulling Dowels

(I’ve) tried everything but the last dowel refuses to come out.

The dowel may have picked up a bit of debris when it was installed, causing it to wedge. Heating it in an oven may help. Your torch provides only localized heat, it gets the surface too hot by the time the heat penetrates to the root of the dowel; heating in an oven allows the heat to soak in.

You can pull any dowel simply by grasping it with a collet-type puller and vibrating the crankshaft with an air-hammer. The usual method is to use to a blunt-nosed chisel in the air-hammer, inserted into the pulley retainer bolt’s thread bore so the blunt-nosed chisel rests against he bottom of the hole. The crankshaft is supported in a padded vise, the collet-puller is tightened onto the dowel pin and pulled firmly by hand as you rap the thing with the air-hammer.

No collet? Then use vise-grips. Dowels are hardened. You can’t mar a properly hardened dowel with vise-grips. Some guys don’t even bother with a collet (i.e., a gripper that grasps the full circumference of a shaft. VW dowels come in three diameters. You’ll need a different collet for each size.). Instead, they take a pair of cheap vise-grips with soft jaws and drill them for the size of dowel they want to grip, less a few thou.

No air-hammer? Then use a regular hammer. Just be sure not to damage the crankshaft by hammering on it. No heavy blows. Pretend you’re an air-hammer.

Why does it work?

I don’t know. Different mass-ratios or something. But it does work . . . all mechanics and machine shops pull dowels this way. Or maybe not all, seeing the trouble your local shop had with it. I’d better make this a public post.

If sleeve retainer was used to secure the dowel-pins you’ll need to heat the crankshaft to at least 450 degrees. Do this in an oven, where you can control the temperature. Let it heat-soak at least an hour to be sure the heat has penetrated to the dowels. After getting out the dowels, put the crank back in the oven, bring it back up to 450 degrees, let it heat-soak about an hour, then shut the oven off and leave the crank in the oven 24 hours or until the thing is stone cold.


(Revision Note: Despite a number of messages from ‘professional’ mechanics saying they’d never heard of a ‘dowel puller,’ such tools are in fact commonly available and every engine overhaul shop usually has one. They are available from Proto, Snap-On and most other suppliers to the trade. The dowel puller usually consists of a number of collets in SAE and metric sizes, which screw into a slide-hammer. When the slide hammer can’t win the dowel free, the trick with the air-hammer is used.)

VW - Used Crankcase

For my rebuild I was questioning whether or not to get a new Brazilian/ Mexican $300 case as opposed to an align bore on a used one. What do you think?

If you can afford it, always opt for a new crankcase.

But don’t read this as a blanket condemnation of all used crankcases. Volkswagen built their factory overhauls on used crankcases, and continue to offer a wide range of replacement main-bearing shells to accommodate align-bored cases and re-ground cranks. The availability of such a wide range of bearing shells makes it obvious that the design philosophy behind the Volkswagen engine expected the crankcase could be overhauled and reused. Verification of that conclusion is reflected by the fact Volkswagen did exactly that.

On the other hand, experts such as the late Gene Berg declared flatly that a used crankcase should never be re-used. This seemed a bit harsh since there were verifiable instances of Volkswagens on RFD routes puttering their way past the 500,000 mile mark powered, at least in their latter days, by factory overhauled engines.

Which makes for an interesting dilemma. On the one hand we have Volkswagenwerk AG with its twenty-million engine’s-worth of experience saying it’s okay to re-use the crankcase, while on the other hand we have race-winning experts saying exactly the opposite. Which one is right? And to add an arrow to the quiver of the ‘experts’, even Volkswagen had to admit that not all of their factory re-manufactured engines stood up as well as they would have liked. Some suffered failures that were remarkably similar to the failures experienced by people such as Gene Berg, failures which justified his conclusion that a crankcase should never be reused. Yet there were those hundreds of thousands of re-manufactured engines which puttered on with absolutely no problems at all. It was very confusing.

As so often happens in life, the answer is not black & white. Both conclusions were valid... under certain circumstances. Unfortunately, those circumstances involved some technical aspects of metallurgy so arcane as to virtually ensure their understanding would remain forever beyond the grasp of the typical Volkswagen owner. Including me :-)

Early Volkswagen engines use a crankcase cast from magnesium alloy. The other principle constituent of the alloy is aluminum and that’s generally as far as anyone bothers to go when defining the metal that makes up the crankcase. But there are other metals as well, including copper, tin, niobium and even iron, albeit in only trace amounts.

Until recently, metallurgists had no idea that metals could display thermally-induced ‘memory’ properties. But once discovered, those properties were used in the space program and elsewhere. To take advantage of the memory properties, which are found mostly in tin/niobium alloys, you create the shape you want then heat the fabricated structure to a certain critical ‘memory-write’ temperature. Once it cools, you may crumple the thing into a ball if you wish, knowing it will return to its fabricated shape when the metal is re-heated.

One of the most interesting aspects of this property is that the ‘memory-read’ temperature . . . the temperature at which the metal will begin returning to its ‘memorized’ shape . . . is considerably lower than the temperature needed to ‘write’ that shape into the metal’s memory. This allowed umbrella-sized dish antennas to be crumpled up to the size of a golf ball and shot into space, where they would gracefully unfold when electrically heated and retain their shape when cool.

It appears that after aging for several years, the magnesium alloy used in the early VW engines could display some of these ‘memory’ properties. No one paid much attention because to get the metal to ‘remember’ a shape, it would have to be heated to well above the engine’s normal operating temperature. Unfortunately, due either to an accumulation of wear or the result of extremely high rpms, as might be encountered in an engine used for drag-racing, some parts of the crankcase could be raised high enough to cause that part of the casting to ‘remember’. If that part of the casting was under stress or distorted, it would ‘remember’ that over-stressed distortion. Apparently, the shape most often ‘remembered’ was multi-lobed oval of a severely pounded-out center main-bearing web.

If my interpretation of this situation is correct, and I want you to understand up-front that it is at best an educated guess, overheating in conjunction with a pounded-out #2 main-bearing web sets the stage for what is to follow.

So you have a crankcase that, except for a pounded-out #2, appears okay. You give it an align-bore and even the most critical blueprinting says you’ve got a good case. But heat the thing to about 300 degrees Fahrenheit . . . considerably lower than the temperature needed to make the metal ‘remember’ . . . and all of a sudden THE CENTER MAIN BEARING BORE IS NO LONGER ROUND!

The problem is not isolated to the #2 main bearing. The oil supply for everything on the right-hand side of the engine passes thru a drilled passage at the #2 cam bearing... which is an integral part of the #2 main-bearing web. Any distortion of the center main-bearing web, either in the bearing bore or in the web itself, will result in a catastrophic loss of oil pressure. The #2 main bearing provides oiling to two of the connecting rods. The web itself forms the oil passage everything else on the right-hand side . . . cam followers, rockers and valves. Without sufficient oil pressure, you really don’t have much of an engine.

Volkswagen eventually changed the alloy of its cast magnesium crankcases. The new alloy has a higher percentage of aluminum, which does not display any ‘memory’ characteristics. And of course, the Type IV is aluminum rather than magnesium.

Now let’s get back to the original question. Should you use an align-bored crankcase?

I often do. But only if I know the provenance of the engine. If the thing has been overheated, or suffered any form of catastrophic failure, I’ll put it aside in favor of a different crankcase.

I’ve built a lot of engines, many for folks who were willing to pay extra for reliability. In those cases, their engine was usually built from all-new components. Over the years I've seen those engines deliver millions of happy miles. As they approached the end of their useful life I had no qualms about align-boring the case, grinding the crankshaft and rebuilding them to spec. Exactly as Volkswagen did with hundreds of thousands of replacement engines. Not racing engines. Bug engines, or bus engines or Ghia engines. Engines fitted with proper cooling systems and full-flow oil filtration.

As to ordering a ‘rebuilt’ crankcase, I can’t recommend it. The odds are, a rebuilt case will do just fine. But occasionally it won’t. If I don’t know the history of the engine I tend to worry. Which brings us back to where we started: If you can afford it, always opt for a new case.

VW - Engine Paint

Engine Paint

In the several years since it was posted my article on painting VW engines has probably produced more mail than any of the other two hundred or so other articles.

That tells me I didn’t do a very good job.

The basic reason for painting your engine to begin with is to protect it from rust and corrosion. But since all paints serve as insulators to some degree, you want to pick a paint that, ideally, will help your engine run as cool as possible.

Within the range of temperatures we’re concerned with, which is basically the maximum range of our oil temperatures . . . say, 400 degrees Fahrenheit as the max . . . a thin coat of flat-black paint will enhance the heat-flow... from surfaces which are in contact with the oil. That means, the valve covers, push-rod tubes, cylinders, generator tower, crankcase (*) and sump-plate.

The physics of this heat-flow enhancement can get a little hairy but they generally fall-in with the reasoning Jazz laid out in his message. Key factors are that the black surface must be thin . . . having to do with the wavelength of the heat-energy being transmitted . . . and must be intimately bonded to the heated surface, the metal to which the paint is applied, and that the paint not contain clay, metallic particles or other substances that act as insulators. In plain language, do not use the so-called ‘high-temperature’ paints, firstly because we aren’t dealing with high temperatures, and finally because such paints act as insulators.

The (*) has to do with aluminum vs magnesium alloy. Paint doesn’t like to stick to aluminum unless the surface has been chemically etched. Since this isn’t practical with the Type IV crankcase, I don’t recommend that it be painted, which is why I specifically mentioned ‘magnesium-alloy’ when talking about painting crankcases. Magnesium is much more chemically reactive than aluminum . . . it is, in effect, ‘self-etching’ (unless passivated) . . . and gains far more benefit from the corrosion-protective qualities of a layer of paint than does aluminum. So paint your early-style crankcase but don’t worry about it if you have a Type IV. (I feel bound to mention that there are such things as self-etching paint intended specifically for aluminum. Most of these are formulated for the aviation industry, are difficult to find except from aviation-oriented suppliers and are expensive. I think such things are beyond the scope of articles directed toward the general population of Volkswagen owners.)

With regard to the aluminum heads, which I also do not recommend be painted, the problem has more to do with the temperatures encountered near the exhaust stacks, which is so high it will destroy all common forms of flat-black paint. There are ways to blacken aluminum and thereby enhance it’s thermal radiation properties . . . you can see examples of this on many motorcycles . . . but the process is beyond the means of the typical Volkswagen owner.

Then we get to the ‘All Black Engine’ confusion.

I trod upon many a toe when I said that folks who chromed their engines hadn’t a clue. That particular thread got its start with regard to the benefits . . .meaning trophies to be won at car-shows . . . of polishing the crankcase.

A polished crankcase, along with chrome valve covers, push-rod tubes, generator tower and sump-plate cause a VW engine to run so hot you wouldn’t believe it . . . the thing literally melts down. Of course, if you live in Lapland, this may be exactly what you want, which is why Volkswagen offered chrome valve covers and push-rod tubes and sump-plates and split bearings . . .all as part of their ‘high-latitude’ package, intended to keep their air-cooled twirler warm and working in a sub-zero climate.

See the problem here? If Volkswagen themselves offered such things . . . and there were part-numbers that would yield-up marvelously well-chromed parts . . .then obviously the things had to be good for the engine, right? Speaking from my perspective in sunny southern Cal, I said ‘No,’ loud and clear. Yet there were those pesky VW part-numbers... Conventional Wisdom wins again.

The truth is, with regard to any part of your engine not in contact with hot oil, you may paint it . . . or chrome it . . . any color you wish. In the case of your shrouding, tin-ware and blower housing, the finish . . . paint, chrome or what-have-you . . . is there only to protect the metal. These metal parts are not a factor in the transfer of heat via radiation. The metal is there to contain the envelope of cooling air. I realize the metal will get hot through both conduction and radiation absorption but the quantity of that heat is minuscule when compared to that being radiated by those parts of the engine in contact with the oil. Indeed, this perception of heat is subjective. When the engine is running and the car is moving, the shrouding and tin-ware is usually only slightly warmer than the ambient air temperature. It is only when the vehicle is brought to a halt and the engine shut off that any significant quantity of heat can be absorbed by the tin-ware. The subjective part is the fact that you can not put your hand on the tin-ware when roaring down the highway at sixty mph... but you can when the vehicle has stopped... by which time the tin-ware feels hot to the touch. And Conventional Wisdom wins again.

Want to polish your crankcase? Chrome your valve covers? Go right ahead. But don’t plan on driving the vehicle.

Finally, those pesky heat-exchangers.... The shrouding of your heat-exchangers . . . the metal canister surrounding the cast aluminum heat-exchanger inside . . . contacts the exhaust system at only one or two points. While the shrouding does get hot through absorption of the heat being radiated by the cast-aluminum heat exchanger, the relatively loose fit of the canister to the exhaust pipe ensures there will always be some amount of air-flow through the heat-exchanger, meaning it seldom gets hot enough to cause the breakdown of regular (as opposed to high-temperature) paint. That means you can paint your heat-exchangers any color you wish. The paint will burn-off in a small area immediately adjacent to the exhaust pipe but the remainder of the metal will be protected . . . and you very definitely need to protect your heat exchangers with a coat of paint, otherwise they will rust out in only a couple of years. The heat exchangers on my ‘67 bug came with the car . . . original equipment. They keep trying to rust, and I keep painting them. I’m sure the rust will eventually win but I think me and the heat exchangers are putting up a hell of a good fight :-)

The heat-exchangers on the Type IV are a different case, one in which I haven’t enough experience to recommend how they should be finished. In my original post on painting your engine I stressed the primary purpose was to protect the metal, to ensure your engine would last as long as possible. The enhanced heat-flow derived from using the proper paint is a freebie but one that should not be scorned through the use of paints or finishes that might reduce the ability of the engine to cool itself.

VW - Loose Barrels

Loose Barrels

I am in the middle of building a 1776cc with 90.5mm barrels. I heard from a friend that the barrels are only supposed to move .5mm max around inside the case. Mine however move around at least 1mm. Will this cause any problems? What could I do besides have the case machined to accept bigger barrels and make an 1835?

You may have a problem. 92mm jugs are made from the same castings as 90.5mm cylinders . . . their skirt and head diameters are the same, only the bore diameter is different. That means your spigot bores are already opened up for 92's . . . and may have been opened up too far.

The spigot hole for cast-iron cylinders in a magnesium alloy crankcase must be kept fairly tight due to the difference in their coefficient of expansion. The normal allowance is about one-thousandth of an inch (0.001") of play for each inch of bore, rounded up, to a maximum of about one and a half thousandth (0.0015"). Since the nominal diameter of the spigot-skirt of a 90.5mm cylinder is 3.785", the nominal spigot-bore diameter would be about 3.790". But those are ‘nominal’ figures. There is considerable variation between the various manufacturers and even within them, with one batch of jugs being a thou up or down from the last batch. Whoever opened up your crank case should have miked your jugs and set their tools accordingly.

Having spigot-holes that are too tight results in hard starting, scuffed pistons and in the worst case, a thrown rod. When the spigot-holes are too loose the cylinders shuffle on the case making it impossible to maintain proper tension on the cylinder-head studs. As they loosen up you lose compression, start losing a lot of oil from the spigot-bores and generally end up with a doggy, drippy, unreliable oil-pumper.

Some lo-buck rebuilders of big-bore engines start with a used crankcase, open up the spigot bores to an enormous 3.825" or thereabouts, slather thick layers of blue RTV on the jugs, slap the engine together and cross their fingers. In most cases the thing survives the warranty period but not much longer.

Since your message did not cite specific dimensions I suggest you start there . . . blueprint what you’ve got and figure out if its usable. You can push the figures a bit . . . maybe three thou too fat . . . but any clearance more than .008" or thereabouts is going to produce the problems mentioned above . . . the bigger the gap, the bigger the problems and the sooner you’ll see them.

A properly built Volkswagen engine is capable of delivering twenty years of reliable service. It’s worth doing the job right.

(Revision Notes: This topic has generated a constant stream of mail, most arguing for greater radial clearance, citing the fact that engines for drag racing often use 0.005" or more of clearance per inch of bore and win lots of prizes.

Which happens to be a completely different subject.

Go find a stock crankcase & cylinder. Measure them. You will find the radial allowance is between 0.004" and 0.010". If you measure a lot of them you’ll find that 0.006" of radial clearance is a fair average.

Over the years I’ve noticed the dimensional tolerance on Brazilian crankcases and replacement cylinders is quite a bit more than it was on German cases & jugs. But that’s of little significance when opening up a case to accept larger cylinders since I always machine the case to match whatever set of jugs I’m using.

You are the Mechanic-in-Charge, not me. You may build your engine any way you wish. I build mine to last.)

VW - Case Savers

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Case savers are threaded steel inserts installed in the VW crankcase to prevent the head studs from shearing their threads and pulling out. American rebuilders of VW engines have been using them since the late 1950's. Volkswagen began installing them in their cases in 1973.

I have seen four different types of case savers intended for after-market installation. The ones I use are threaded 14 x 1.75 (exterior), thru-threaded on the interior for either 8mm or 10mm studs. Variations include those which are closed at the bottom and those having different exterior threads but 14mm x 1.75 seems to be the most common. (The earliest ones I used had an SAE exterior thread.)

Case savers are installed as a matter of course by most overhaul shops. If building a large displacement engine using an early crankcase you will want to select a case saver that will not interfere with opening the spigot bores for larger cylinders, nor get in the way of relieving the case for stroker cranks.

Installation is a straight forward drilling & tapping job. Special tooling is used to support the left case half (i.e., the one with the main bearing studs). Tapping is done with a Tap-matic sensitive feed or by hand. The case-savers are normally installed with high strength, hi-temp loc-tite and allowed to cure before any crank relief work or machining the spigot bores. To thread the case savers into the case I modified a couple of old spark plugs, fitting them with 8mm and 10mm stud-ends to serve as installation tools. (If no one is watching, I run them in with an air tool.)

When properly done, installation of case savers is a one-time job that eliminates the possibility of pulled studs. Since case savers are nothing more than threaded sleeves, anyone with a lathe can make them. Although seldom advertised, case savers are available from VW after-market suppliers such as Johnny’s Speed & Chrome, Barrett Enterprises, or Hoy-Fox. Cost is about forty cents each; you’ll need sixteen.

Case savers, often listed as ‘stud inserts,’ are superior to Heli-coils due to their larger contact area and are used in aircraft engines where maximum strength is needed.

VW - Case Paint

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What kind of paint should I use on the crankcase?

Over the years I’ve used a variety of different oil-based paints. With the exception of flat black primer, most stood up fairly well. Oil-based paint is usually okay up to about 400 degrees Fahrenheit and since the crankcase is never hotter than your maximum oil temp, that gives you a wide margin of safety.

My favorite is Rustoleum Flat Black. You want the flat-black because it has a better heat-tranfer index than glossy. If all you can find is Gloss Black, simply cut it about 1:4 with unleaded gasoline(!) or naptha. That causes the paint to dry dull instead of glossy.

Avoid ‘high temperature’ paint at all costs. Such paints get their high-temp properties from clay or metallic salts, both of which make excellent insulators.

VW - Slip-in 88's

Slip-in 88mm ‘big bore’ jugs

Don’t bother. ‘Slip-in’ 88's are a sucker-bet, intended for the ‘kiddie’ trade.

Slip-in 88's are in fact stock 85.5mm jugs that have been over-bored by 2.5mm, which means the walls are thinned down by 1.25mm or about fifty-thou. The resulting sealing surface is barely a tenth of an inch wide and experience has shown that simply isn’t enough to maintain a reliable compression seal.

Indeed, the stock 1600 is little more than a slow leak, compared to the 1500, which in turn was a notorious dripper compared to the stone-reliable, leak-free 1300. Which is no mystery because the jugs in the 1600 started out as the 77mm jugs on the 1200 engine :-)

Increasing your displacement is always the most reliable means of increasing the engine’s power but that assumes you don’t break any rules along the way. Slip-in 88's are little more than a built-in headache, resulting in compression leaks, warped barrels and scuffed pistons. Enormously popular among the kiddies, of course, giving them bragging rights to a ‘big bore’ engine.

-Bob Hoover

AV - Riveting 101, Part 3

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Got your gun? Retainer? Rivet set? Regulator? Bucking bar?

Do you realize how lucky you are? You picked up the phone, made a few calls and all this stuff appeared on your doorstep in less than ten days. Not very expensive, either... assuming you've bought used stuff... which is fine for learning.

Now try that in Old Patagonia.

In effect, America's aerospace industry has subsidized the tools and materials you need to build a metal airplane. That's the lucky part. Ding your bird in the boondocks of the world and it's liable to stay there even if the required repair is relatively minor, because the tools and materials and skills simply are not available.

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Okay, lemme ask you again: Do you have your gun? Rivet set? Retainer spring? Regulator? Bucking bar?

Have you got any idea in the blue-eyed world what I'm talking about?

Early on, I suggested you obtain copies of the catalogs from Aircraft Spruce & Specialty Co, and from U.S.Tool. One of the reasons for that suggestion is that the catalogs serve as excellent TEXT BOOKS when it comes to defining rivets and riveting equipment.

I also made mention of the enormous variety of riveting methods and equipment, and of the procedures specific to using each of those equipments effectively. The catalogs will give you a better understanding of what I meant.

Rivet guns are sized according to the diameter and hardness of the rivet they can set in a given amount of time. This is a pretty rubbery classification since by adjusting the air pressure you can drive a small rivet with a big gun, or a larger, softer rivet with a small gun. For general maintenance or building a homebuilt, you should do fine with a 2X or 3X gun.

The major difference between a pneumatic hammer designed for riveting and one designed for cutting off mufflers or whatever is in the trigger -- actually, in the valve the trigger actuates. For riveting, you need perfect 'throttle control'. This usually involves a valve having an orifice that varies in a uniform fashion as the valve is opened. On cheap 'chatter guns', such as the stuff sold by Harbor Freight and others, the valve is a simple hole, an on-off switch offering virtually no control at all.

There is an almost endless number of rivet sets, the variety dictated by the need to set rivets in corners, the bottoms of channels, behind stringers and so forth. For our purposes you'll do fine with a straight 470-4 set, 3-1/2" long. Used, expect to pay about three bucks, new about five. (But to add a whiff of reality... lightplanes typically use -3's, -4's and a few -5's... so you might as well order a whole kit of sets if you're serious about riveting.)

Rivet sets for guns have a standard shank size -- .401" The set slides into the barrel of the riveting gun and is held there -- loosely -- by a retainer in the form of a coiled spring. Retainers come in two flavors, Beehive and Quick-Change. I'm not as quick as I usta be and I've got several guns so I tend to use the beehive type.

Being springs, retainers tend to wear out or break fairly often so ALWAYS keep a spare in your kit. And if you use the quick-change type (often abbreviated QC), be sure to buy or make yourself a Quick-Change lever... or you'll find changing sets to be not quite as quick as you'd think.

The regulator screws into the air-inlet on the gun. It is basically a valve. Some guns have them built-in, some don't. The external regulators tend to be a bit more accurate than the ones which are built-in so it's customary to use the external type.

It is also common practice to fit all of your air-tools with in-line connectors. Harbor Freight carries good-quality brass connectors at a very attractive price. You put the female on your hose(s) and the males on your tools. Be sure to use a dab of Permatex or similar sealant when you thread the fittings into place.

Bucking bars are the hand-held anvils against which you upset the rivet. Their basic graduation is by weight, with a pound and a half being about the lightest that will prove usable in regular work. The heaviest commonly available buck is around nine pounds but you don't see them very often. The general rule is that the larger/harder the rivet, the heavier the buck. For general repair work and homebuilt construction, your heaviest buck will be on the order of three pounds with a one and a half pounder probably being used for about 90% of your work.

Other than weight, bucks are shaped to allow them to access the shank of the rivet to be upset... which means they can take virtually any shape at all. Indeed, for some repair work the first task is to manufacture a buck that will allow you to do, by hand and in the field, what the factory did on the bench with automatic tooling.

High quality bucks are forgings having polished, induction hardened faces. But they remain just anvils -- a mass of metal. You can make your own bucks (and will make at least some) from mild steel, suitably polished. If you're familiar with Kasenite or other surface-hardening compounds -- or if you're a real machinist and know about case-hardening -- it's common practice to harden the working face of locally fabricated bucks. And of course, the working face is always polished. Whatever surface finish on the face of the buck, it will be transferred to the shop-head. Smooth is good. Anything else flirts with rejection since relatively minor surface scratches may be seen as stress-risers. Fortunately, there's a simple test for the surface quality of your shop-heads: They should look like mirrors. And continue to reveal a mirror-like smoothness when examined with a 3x glass.

Okay, last time... Got your gun? Rivet set? Retainer? Regulator? Bucking bar? Then let's go to work.

Got a block of wood? Got your ear muffs on? Safety glasses? (Shit happens. Don't mess around.) [I use those Lexan things that fit over my regular glasses.]

Give your gun a drop of 3-in-1. Make this a habit. Do it a couple times a day during the course of riveting. Most guns don't wear out, they rust out. Don't waste money on an in-line oiler or other fancy stuff intended for the factory floor, they'll only contaminate your hose, rendering it useless for painting. Instead, spend some time making good WATER TRAPS for your air supply. Commercial air-tool oil is primarily a rust preventative, lubrication is only a secondary function.

Insert a 470-4 set into your gun (ie, a set that matches your rivets) and secure the retainer. Now press the set against the block of wood (flat on the bench, please) and gently squeeze the trigger. You should hear a slight hiss... (awright! Stop fooling around and turn on the air compressor!)

A gentle squeeze of the trigger should produce a measured 'thunkathunkathunk' from the gun. If you got a rapid 'brappp!' it's getting too much air. So dial it down. When just lightly touching the throttle, thunkathunka is good, brappp is bad.

Thunkathunking okay? Then squeeze harder -- open up the throttle a bit more. The gun should provide a SMOOTH response from low speed to high. And yes, the high speed will sound like a 'brappp!'

No brapp? Then check your line pressure. The gun requires a given volume of air per blow or cycle, whereas the cyclic rate reflects the amount of pressure behind that volume of air, as in how fast it can re-cycle. In adjusting your gun you are taking into account your compressor's output, the size of the gun, the type of work you'll be doing and any restrictions that might apply to your air supply, such as a long run of small-diameter hose. (In the latter case, the usual fix is to place a tank -- a 'local reservoir' near the work, plumbed to the compressor via a check-valve. You may then leave the compressor at some distant location and still have adequate pressure & volume at the gun. You can make yourself a local receiver out of a portable compressed air tank, the sort of thing used for filling a flat tire. Handiest place to find a suitable non-return valve is from an auto-body paint supplier, whose rice-bowl revolves around compressed air equipment.)

- - - - - - - - - - - - - - -

Here's a Rule. Violate it at your peril.

NEVER trigger a rivet gun UNLESS the set is firmly placed against... something, be it a panel you're working on, a block of wood or whatever.

They aren't joking when they call it a gun. And retainers tend to snap when over-extended.

- - - - - - - - - - - - - - - - -

Once you've got your gun adjusted, put the block of wood in your vise so's it's sticking out to the side, like a driver signaling for turn. Got your bucking bar? Okay, hold the bucking bar against the back of the block of wood, put the rivet gun against the front and trigger the throttle. Do it a couple of times.

Now think about it.

The gun needs to be perfectly perpendicular to the block of wood. And the face of the buck must be perpendicular to the axis of the rivet-set. Adjust your stance as needed. No, you can't lean over like a willow in the wind, it'll throw off the geometry of your arms. Try to keep the mass of your upper body more or less centered over your hips. Stance is important. As with golf or marksmanship, a proper stance skews the odds of success in your favor.

Try it again. Visualize shooting something from the gun straight through the block of wood into the bucking bar. Get the feel for the proper alignment in your hands and arms and feet. You should have a sense of being poised but stable. As with drilling, you are not doing the work, the tool is doing the work... you're only there for guidance. You aren't pressing the gun against the wood with any great amount of force, nor are you pressing the buck toward the gun very strongly. 'Firm' is the best definition I can offer but you will have to translate that into your own terms... and the rivets will tell you how well you've done.

Now go make up some coupons. Flat work only, for the time being. (And yes, you can make them any size you wish... and with as many holes as you wish. But five or six rivets at a time is more than enough in a training environment.)

Make up your coupons complete, right through to insertion of the first rivet. Position a coupon in the vise in the same orientation as with the board. Position the set... then the buck... now feel how the set indexes on the head of the rivet.

Initiate the hammering with a gentle squeeze, allow the hammering rate to increase -- but not up to a full-blown 'brapp' -- and STOP. Should take less than a second.

Don't move. That's the 'follow-through'. The set should still be perfectly indexed on the rivet, the buck still perfectly centered and YOU HAVEN'T MOVED... except for your trigger finger. (Why the 'follow through'? Because of inertia. You aren't aware of it but your MUSCLES have become tense as they did the work. They have balanced the thrust of the gun against the inertia of the buck. If you allow yourself to move in an uncontrolled fashion, the set will come off the manufactured head and may go galloping off across the panel... or the reflexive reaction of your bucking-arm may cause you to bend the panel.... or some damn thing. The key point to be learned here is that a form of 'follow-through' is required. What you call it... and how you deal with it... is up to you.)

Okay, inspect it.

The whole story -- everything you need to know about your technique and the setting of the gun -- is right there in that one rivet. And you are the Mechanic-in-Charge, the man on the scene.

Dead soft rivets -- 'A's -- are about as hard as butter. Odds are, you hit it a little too hard... the shop-head is 'pancaked' -- too wide and too thin. So here's your options: You can try a lighter buck or you can lighten up on your touch... not hold the trigger down so long... Or you can reduce the air pressure... (I prefer to adjust the pressure; I use about the same 'touch' for all rivets. But it's a personal thing... do what feels best for you.)

Whatever you decide to do, do it. Then go back through this cycle. And keep doing it until you start seeing nice, symmetrical buck-heads.

If one comes out too high, don't go back and hit that it again. Do a new rivet. Why? Because you are learning. At this stage of the learning process the object is not to produce a perfectly set rivet, the object is to learn how to accomplish that feat. Going back over your work simply adds noise to the signal at this point. The goal is to learn how to head the rivet with one smooth brappp.., rather than how to do it with a series of brappp's.

The rivets themselves provide the history -- the feedback -- needed to interpret what you are doing wrong so that you may eventually do it right, every time. In this process, the rivet is secondary to the process of riveting. You are trying to master the process. The rivets are just a means of keeping score, as it were.

The significance of this learning process escapes many people because it runs counter to the modern-day philosophy of 'Everyone's a winner!' as well as the modern-day definition of 'education', which has virtually nothing to do with true learning. Riveting is a senso-kinetic skill. It isn't something to be memorized and regurgitated up during Finals, you must train your muscles to accomplish the task without direct, real-time feedback from your brain -- your nervous system simply can't react fast enough... just as when learning to ride a bicycle.

Another reason this point escapes a lot of people is because they don't understand that in the process of acquiring information -- which, upon integration becomes what we call 'knowledge' -- all data, negative as well as positive, is valid data. Indeed, you must have both or you can not define either. ("Okay, that's what we call 'falling off the bike.' That's bad. Now get back on and try it again, except this time try not to fall off.")

So make your mistakes. Make a lot of them. And in making them, learn from them. There is no 'failure' as such when you have done the best you can and learn from your mistakes. But if you tackle a task with the assumption you must do well from the outset -- that to do less makes you a loser -- odds are you lack the required attitude to do well at anything.

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So when do you go back and give a high rivet an extra brappp? When you know the first hit wasn't enough. You've hit it. Your muscles and your brain recorded the hit. Now the little man in your head sez 'Hit it again.' And you do what the little man sez.

The variation on this theme is when you are part of a riveting team. The Captain of the team is the bucker. He or she is often out of sight on the other side of the panel. You communicate by taps of the buck against the work. The usual code is one tap to say 'Hit it again; not fully headed', two taps sez it's good, move on to the next. Three taps is telling you to drill the sucker out and drive another. Which brings us to a crux in the art of riveting...

Wanna know the real secret of riveting?

It's drilling them out.

Drilling out dead-soft rivets is pretty much a waste of time. Oh, you can do it.. and you're gonna have to learn how. But A's aren't the best stuff to use when introducing you to the process. For that, I'd like you to obtain some AD's.

To learn how to drill out rivets you make a test coupon about eight inches wide by a foot long out of half-hard aluminum. On the bench, or using a drill press, layout and drill six rows of ten holes. Now fill each hole with a properly set rivet of suitable diameter and length.

Then drill them out... without damaging the coupon.

In the overall scheme of things the ability to drill out a rivet will always have a greater value than the ability to properly set a rivet. The reason is pretty simple: Even if your riveting skills are rather poor, so long as you can drill out rivets without damaging the part, you will eventually produce an acceptable finished product. (Bad rivetors are often referred to as 'blind hogs', apparently from the old saw: 'Even a blind hog finds an acorn now and then.' Of course, in the peace-time reality of today's world, a bad rivetor is more likely to be referred to as 'unemployed' :-) More important, in the real world, airframe maintenance involves repairs... and you can't do the repairs until you remove the rivets fastening the bad part.

To drill out a rivet you use a drill-bit that has a smaller diameter than the shank; typically you would use the pilot drill-size (ie, a #31... or even #32 for a 1/8" rivet, for example) to drill through the head of the rivet... and a few thousandths beyond. You then insert the shank of the drill bit or a punch of suitable diameter, into the drilling and gently wobble the head until it breaks off. The shank of the rivet may then be punched out. There are a host of precautions that must be observed when drilling-out, such as supporting the panel when punching out the shank and being sure your hole is properly centered.

It's vital that you drill in the exact center of the rivet. On AD's, the signature dimple often remains visible and may be used as the center. On others, it's usual to file a small flat on the head and use a center-punch. For flatheads, there's a little optical device made out of Lucite that allows you to center the rivet in the bullseye of the device then flick the spring-loaded punch, marking it with a high degree of accuracy. For rivets having raised heads there are a number of tools devoted to drilling-out, most of which will automatically center your drill-bit, limit the depth of the hole and prevent the rivet from spinning, all in one operation.

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To add to the utility of this article, which I imagine is a rather boring read for the more experienced hands, I'm included drawings of a fairly typical Apprentice's Toolbox. Thanks to Mr. Ryan Young, you'll find them posted at...

http://users.lmi.net/~ryoung/Sonerai/Toolbox/Toolbox_Warmup_Project.htm

As a point of interest I wrote ‘Riveting 101' several years ago complete with drawings, photos and assembly instructions for the toolbox. The article was submitted to several magazines, who either rejected the material or offered only a few cents per word. (Good magazines pay a dollar a word and up for illustrated technical articles.) One aviation rag wanted to print it without any payment at all :-) Since that time the assembly instructions and photos have gone adrift. If I find them I'll send them to Mr. Young to tack onto the drawing file.

The only tricky bit in the construction of the toolbox is offsetting the corners by the thickness of the panels. That is, the panels do not overlap each other at the corners. This allows the builder to use whatever thickness of sheet stock they happen to have on hand.

To demonstrate their use, the tray was assembled with flush-head rivets, some dimpled, some counter-sunk.

Wooden dowels, 3/4" in diameter, may be used for the handles in lieu of aluminum bar stock.

To make the box waterproof buy a small tube of ‘Boat Life' polysulfide caulk and treat the edges of the panels prior to assembly. You may pretend it's ‘Pro-Seal,' if you wish :-)

-R.S.Hoover, 9 September 2002

AV - Riveting 101, Part 2

So... you've got some coupons and you got some rivets.

Down the middle of one of your 2x6" coupons, drill a series of five #31 holes. (No, not like that. You've gotta back-up the metal with something. Here... put it on this piece of 2x4. Hold the drill-motor vertical. No, don't push on the thing! The weight of the drill-motor and the sharpness of the bit is all it takes to make a clean hole. Okay... now you got it. Go ahead and do the other holes.) [If you're using a punch you're about to discover it's principle limitation. Normally, you use a punch to create holes in a part when doing flat-work on the bench. When it comes time to fasten that part to some other part, the matching holes are usually drilled. But for the purpose of this exercise, if you don't have a drill but do have a punch, stack the coupons and punch both at the same time.]

Gottem drilled? Now feel the holes.

Feel that turned edge? That's the infamous 'burr'. Shot Hamilton in the gizzard and completely upset the course of American history.... (Why are you all looking at me like that? Oh! Sorry... I thought it was Thursday... [Aviation Sheetmetal... Mon-Wed-Fri. American History, Pre-Civil War Era, Tue-Thur])

To debur the holes take a 5/16" drill bit in your bare hand, hold the coupon in your other hand and press the drill-bit lightly into the burred hole and give it a twist. You want to remove the burr without chamfering the hole. (A little harder... Not that hard... ) Use your fingers as your gauge -- feel to see that the burr has been removed.

Deburring with a drill bit assumes you're using a bit with the standard 118 degree included angle. You may also use a fine tooth single-cut file, with the understanding that you'll only do so when the hole is so positioned -- and the file wielded in such a manner -- that you won't scratch the surrounding metal. The popularity of 'patent' deburring tools is that with some of them, you can debur both sides of the panel at the same time (!). This allows you to debur holes which open into voids or other spaces where you can't get at the other side.

Now take a second coupon and drill a matching line of holes. Call when you're done, I'll be outside copping a smoke.

Didn't work, did it :-)

Unless you've got a CNC computer screwed to your ass there's no way in the world to produce two matching rows of holes using only a hand-drill. Unless... Did you try using the first coupon as a template? Drilling thru it? Ummm... slipped, eh? Okay, let's try clamping the two coupons together. You can use a C-clamp, a G-clamp, a spring-clamp or a pair of visegrips. Of course, it'll probably bugger up the metal. Buggering up the metal with clamping marks is considered bad form since it can make the plane crash. (So you glue little cork pads to your clamps, or buy the real thing, which comes equipped with little nylon pads.) So go ahead -- clamp the coupons together and try again.

By the time you've drilled the third hole it's becoming obvious that clamping the edge of your coupons only works for one or two holes. After that, they start to wander. (Edge clamping will work... if you clamp the entire edge. This works well for match-drilling small parts. Simply fit your bench vise with soft jaws and clamp both parts in the vise.)

What you need is some form of clamp that centers on the hole itself!

This need became evident in Roman times, when rivets were used to join panels of armor. By the late 1700's when boilers arrived on the scene, metalsmiths already had a variety of tools for positioning and aligning rivet holes. Most of the tools you'll be using for aircraft work are simply modified forms of earlier tools which accomplished the same task. Most of those aviation-related sheetmetal tools were developed in Germany in conjunction with airship fabrication, the frames of which were Duraluminum (yes, it's a proper noun although not in common usage). Both Duraluminum and all-metal airplanes originated in Germany.

Try using a sheet-metal screw. Go on; try it. Number six sheet metal screw, half an inch long... goes into that #31 hole a treat and takes a fair grip on the two coupons as well. Of course, the head of the sheetmetal screw is going to leave a mark... (Another trick is to use the sheetmetal screw to fasten the parts to the bench-top(!) or backing-block (ie, the thing you're drilling against). This is perhaps the most common method when drilling and works equally well using clecos.)

Feller named Parker came up with the solution for the scratches. Special sheetmetal screws having a little rubber or nylon washer made right onto the screw. Manufactured by the Kenton company (Kelon? Something like that). We called them P-K's. Handy.

Well... sorta handy. You gotta screw the thing in... then you gotta screw it out again before setting the rivet. And sometimes the screwdriver slips and you scratch the panel... which ruins the whole thing. So they made hex-head P-K's. Run them in with a nifty little socket-tool, takem out the same way. Of course, that's an extra tool to keep on your belt. And those threads do bugger up the hole a bit. But they really do keep the parts aligned and don't scratch the work. (Hint: A piece of masking tape makes an adequate temporary washer.)

Nobody uses P-K's any more... unless they need to. But the use of threaded temporary fasteners is a valid procedure and was the industry standard for more than thirty years, before being displaced by the more useful (but far more expensive) spring-type sheetmetal fasteners introduced by Cleveland Equipment Company. The use of P-K's... or sheetmetal screws and masking... remains a valid method for the homebuilder... and for doing emergency repairs when you don't have a lot of clecos handy.

(Pop Quiz #2: Why do we call 'cleco's' clecos? Hint, hint... read the last paragraph :-)

P-K's will allow you to match-drill the second coupon. Then you remove the P-K's, take the coupons apart and deburr the second set of holes. At this time you'll notice that the P-K's have left a burr on the original holes(!) Some days it don't do to get outta bed, eh?

Okay, put the two coupons back together using your sheetmetal screws which we are calling P-K's. Chuck a #30 bit into your drill-motor and lets rivet that thing up.

Of course, you gotta figure out which rivet you're going to set first. I mean, you got five holes, right? Where you gonna start?

Ever stitched up a wound? (Don't look like that. It's a handy skill to have in your warbag. Ditto for knowing how to give shots and starting an I.V.) To close a wound with stitches you generally start in the middle. The idea is to keep things nice & even. You do the same thing when you're setting a row of rivets. So remove that middle sheetmetal screw and drill the hole to rivet-size with your #30 drill. Any burr? If so, remove it. Now insert the rivet, position the assembly on your anvil and use a hammer to form the shop-head.

Hey! Nice job.

So keep doing it. Remove the screw to one side of your rivet, drill, deburr and set another rivet. (Hey! This is going pretty good!)

Now go to the other side of center and do it again. Then back, then back again.

Five nice neat rivets in row.

Okay, so they aren't all perfectly symmetrical. That will come with practice. And that's what I'd like you to do -- practice. Do at least five pairs of coupons. You don't need someone looking over your shoulder, you can tell a good set from a bad one; just do the best you can. And the more often you do it, the better they'll become. (Don't sweat the mistakes. We all make them at first. Now's your chance to make all the mistakes you want, the coupons ain't gonna fly.)

Working with coupons and P-K's, you will have seen how the sheetmetal screws touch the anvil when you try to head the rivet. The usual solution is to use a narrower anvil. But in the real world there are riveting tools called 'hand-sets'. These are round or retangular steel bars having a polished face that is machined to match the head of the rivet you're setting. In use, you can clamp the hand-set in a vise and use it to support the head of the rivet while you form the shop-head with a hammer. Or, you might turn the work over, pressing the shank of the rivet against the anvil and hammer on the end of the hand-set. Hand-sets are fundamental to riveting. I'll have a bit more to say about them later.

Working with two flat coupons, the riveting is very easy. But most aircraft components aren't flat. Take a coupon and bend it length-wise -- turn it into a six-inch long piece of angle stock. (Just clamp it between something and fold the edge over with your hand.) Now you may call it a stringer, if you wish :-) Drill five holes down the center of one of the flanges then go ahead and rivet it to another coupon.

The flange limits your access and will make the riveting more difficult. You may find you need a hammer with a smaller head. Indeed, the head of the classic riveting hammer is made from hex or square steel and is typically about 3" long from the handle-hole to the face. As with rivet-sets and bucking bars, riveting hammers came in an almost infinite variety, each shaped to allow the riveting of a particular shape of panel or stringer. The same is true for their matching hand-sets.

But eventually you simply run out of room -- you can't get at the rivet with a regular hammer. For example, take a coupon and bend it into a 'U' about an inch across the bottom. Now figure out how to set a rivet with the shop-head in the bottom of the 'U'

If you're like most folks, you will have reached for a drift or flat-faced hand-set that is long enough to reach the rivet in the bottom of the 'U'. But unless you have a third arm and prehensile tongue, you also discovered that the parts were more difficult to hold and position than in previous examples.

You've already seen the need for positioning and locating the components prior to riveting. Working with angles and deep 'U' channels should bring home the need for some means of positioning and holding the work. 'Relative to what?' ...someone always asks, which is a good question, given the three-dimensional nature of aircraft structures. Do you position the work relative to the anvil or relative to the hammer?

The most correct answer is that you should try to position the work relative to YOU --- to the person doing the riveting. You will do your best work with both feet on the deck and the panel at work-bench height. And yes, this is a bit of a trick question because up to now the anvil has been fixed in position and we've been positioning the work atop it. But the anvil -- the mass against which the rivet is 'bucked' or deformed -- does not have to be fixed in position. So long as it has sufficient mass, we may even hold it in our hand and do a good job of forming a shop-head, thanks to the laws of inertia. But before freeing you from the bench, let me tell you a bit more about hand-sets.

Hand-sets used to be like mother's milk to an aviation metalsmith. But with squeezers and pneumatic guns having become ubiquitous in the trade, the traditional hand-set has largely vanished, replaced by a simple block or bar of steel, drilled to accept the die from a squeeze-set or rivet gun.

The sets for squeezers look rather like a rivet, being a button of steel with a shank extending from the bottom. The top of the button is shaped to match the head of a rivet and is normally hardened and polished. The shank, which comes in various standardized sizes to match the wide variety of squeezers and tools, is three-sixteenths (actually, .187 ) in diameter for small tools (and smaller rivets) and goes up to three-eights (.375) for the larger. Among that range, as a homebuilder, you will probably find the three-sixteenths sets (ie, .187" shank diameter) to be the most useful.

To make yourself a hand-set you simply chuck a suitable bar of steel into the lathe and drill a #13 hole about five-eighths deep in one end. If you were an apprentice, you would be expected to knurl the thing and polish the hammering face but in the real world you will find yourself creating hand-sets as needed, from whatever stock is on hand and in whatever shape is needed to insure the rivet is properly headed.

Which brings me to perhaps the most important bit of information you're liable to get from this exercise: Most rivets are not set in what Conventional Wisdom insists is the 'traditional' fashion using a rivet 'gun' and bucking bar. The overwhelming majority of rivets are set using various forms of squeezers, or in the case of homebuilders, are headed by hand. The pneumatically powered riveting hammer and bucking bar is largely reserved for what is called 'panel work', where it's impossible to use a squeezer... (although aircraft manufacturers have built squeezers so large the entire panel can be pushed through them).

The popularity of squeezers and presses over guns & bucking bars is well justified. A squeezer takes no special skills yet produces a shop-head of perfect proportions and uniform quality. Since the object of riveting is to create the best possible fastener, anything that promotes that goal should be used.

If you haven't already realized the obvious, let me say now that I can't teach you to rivet via email. Indeed, I can't teach you to rivet even if we were in a classroom together. Riveting is a senso-kinetic skill, akin to riding a bicycle. Riveting, like riding a bike, is something you can only learn for yourself. What I'm hoping to do with these articles is to show you the natural progression which lead to riveting as we know it today. That progression begins with forming the shop-head by hammering directly on the shank of the rivet. The next step is to form the shop-head using a pneumatic rivet hammer and bucking bar. According to Conventional Wisdom that would appear to be the logical goal of any treatise on riveting but the reality of riveting is that most of it does not involve the use of rivet guns and bucking bars. Explaining why this is so and introducing you to some of these extremely valuable tools and techniques, is the ultimate goal of these articles. The fact that you might just happen to learn to rivet along the way is merely frosting on the cake :-)

---- to be continued ---
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