Saturday, July 14, 2007

Crank Basics - III

(Be sure to read Crank Basics I & II)

Crankshafts from China, supposedly forged, supposedly of SAE 4340 steel, having been coming into the marketplace for some time now at prices ranging from under $200 to over $400. Same cartons with the same cranks inside them but from different retailers.

Inside the box you'll find the crankshaft wrapped like a mummy. The plastic wrap and cardboard box is its only protection against the rock & roll of an ocean crossing, to say nothing of the hazards of UPS, should you order one. (Stock, replacement crankshafts from Europe are shipped a molded styrofoam block.)

The plastic wrapping is in the form of a sheet. The crank is first rolled in a bundle of the stuff, the ends neatly wrapped in upon the crank, then thirty feet of the stuff is spun out like a pig's intestine to make a plastic rope, which is bound around the mummy.

Your first chore is to unwrap the mummy without dropping the thing on your toe. Unwrap because cutting this stuff is not only a thankless task, it leaves a hell of a mess. So try unwrapping before reaching for your sword.

Once unwrapped the mummy turns out to be a fully machined, counter-weighted Volkswagen crankshaft fitted with eight dowel pins in the Porsche/SPG pattern.

Using a new gland nut as a gauge, you should check the threaded bore in the flywheel-end of the crank. Ditto for the pulley hub, using a clean pulley-hub bolt. Use new Woodruff keys to gauge the keyways milled into the nose of the crank. Finally, use a snap-ring to test the width of the snap-ring groove.

As a point of interest the crankshaft shown in the photos weighs almost exactly 21 pounds whereas a stock crank weighs about 16-3/4. Longer rods and bigger jugs also weigh more than their stock cousins but the weight difference between bone-stock and the largest big-bore stroker is rarely more than ten pounds.

After cursory checks of the threads, keyways and weight, suspend the crankshaft and perform the ringing test for cracks. (Just give it a light tap with a small hammer. It should ring like a bell, where a cracked crank gives a dull clank.) Using a good light and a magnifier, if necessary, give the crankshaft a close visual inspection paying particular attention to the radius of the corners and the lips of the oiling holes. A simple radius gauge filed out shim brass will prove handy here. (The specs for radius and dimension are printed on the form mentioned below.)

In the Chugger's Group files archive, in the crankshaft folder inside the Engine file, you'll find a Crankshaft Check Form. Print a copy, add the date, the serial number of the engine and any details about your micrometer that may be germane. As with all professionally built engines, the blueprint check-list will become a part of the engine's documentation package, along with a copy of the crankshaft's bill of sale, balancer's report and magnaflux report. The package need not be elaborate but it must be complete.

A set of standard bearings will serve as a quick check for main bearing journals 1, 3 and four.

With bearings 1 and 4 installed on the crankshaft you can use one of the crankcase halves to support the crank while you measure the run-out at the #2 bearing. The alternative is a good surface plate and vee-blocks.

To mike your crank you'll need to make a minimum of two measurements per journal and record the results. Provide yourself with good light and sufficient time; this isn't something you want to rush through. (The magnifying glass is because I often forget my eyes are nearly seventy years old :-)

Once you've miked the crankshaft, stand it on its nose using a spare pulley then bag the thing. A large zip-loc bag can be tightened around the nose of the crank to provide a dust-free environment suitable for a few day's storage. For any longer period the journals need to be protected with grease.

If you're satisfied with the crank's dimensions the next steps are to have it magnafluxed and balanced.


The crankshaft shown in the photos was a bit of a disappointment. Main journals 2 and 3 miked 2.1642", which is the low end of acceptable specs, while #1 miked 2.1640" -- two tenths below spec. Assuming it magnafluxes okay it can be used but the bearing clearances are going to be more than I like. It will definitely be happier with 40W rather than 30.

The final step before cleaning & use is to attach everything that will rotate in the same plane as the crankshaft and deliver it up to a competent balancer.


(Ed. Note: In case you were wondering, this is not the same crankshaft as shown in Crank Basics - II.
These crankshafts are often advertised as being balanced and magnafluxed, made from high-alloy steel and so forth. Yet none of the two dozen or so I've examined have shown any of the marks characteristic of balanicng. )