Saturday, December 2, 2006

VW - Cracked Head

> Besides using a compression test or disassembling the engine, what would and
> engine sound like or do if a head was cracked?

You've got things a bit backwards there.

The crack itself, which is quite common on dual-port heads, has no particular characteristic with regard to engine sound or performance. It's what comes next that gets your attention. :-)

The cracks typically form between the valve seats, and from the valve seats to the spark plug hole. Once a crack appears, it ALWAYS gets progressively worse; cracks do not 'heal' themselves.

The 'what-comes-next' is the exhaust valve seat, which comes loose from the head. This generates all sorts of unique noises that an experienced VW mechanic can recognize from a block away. Most describe it as a 'tinny rattle' or variations on that theme but no two people will perceive it exactly the same.

Mechanically, a loose valve seat will usually cause the valve's lash to CLOSE and in doing so, offer a significant warning well before the grand finale. Normally, valve lash becomes wider in use. The only times it becomes LESS is when the valve begins to stretch or a loose valve seat is be being hammered into the head, either of which is portent of pending disaster, well worth the effort to drop the engine and sort things out.

Most don't, of course. Most continue to drive the poor thing until they've converted the relatively minor repair into a major disaster requiring the engine be overhauled or replaced.


So why do heads cracks?

There isn't one sole reason but a combination of things. Bad design is a major player. When VW went to the dual-port design it had to make the casting thinner in several areas to gain room for the second port.

Metallurgy is a significant factor. Aluminum is what's known as a 'white short' metal, a term from the casting industry that describes metals which become frangible when heated. Heat cast aluminum to its frangible range and it breaks apart like a cube of sugar. The frangible range for cast aluminum depends upon the particular alloy but for VW heads it begins at about 450 degrees on the Fahrenheit scale and extends up to about 1200 degrees, where the metal begins to melt.

If you want to crack a VW head simply raise its temperature to within the frangible range then subject it to shock or stress. The manner in which the valves snap closed is enough shock to initiate cracking in an overheated head, which is why most cracks begin between the valve seats. But detonation will also do it and since detonation is an artifact of overheating, it's possible to trash an engine quick like a bunny just by losing the fan belt or keeping your foot in it too deeply on a hot day. The stress produced by uneven torque on the head-stays is another common cause of cracked heads.

How the vehicle is driven and maintained generates a whole file full of good reasons for VW heads to crack but tearing out the thermostat tops the list, followed by a loose fan belt, missing tin-ware -- especially those 'unimportant' pieces the experts tell the kiddies they don't need :-)


So how do you prevent head cracks?

With dual-port heads, you can't. The design is inherently flawed. Given enough time, cracks WILL form between the valve seats and once the crack has started, it will continue to get worse.

Proper maintenance of the engine, especially with regard to its cooling system, will stave off the on-set of cracking but even the most cursory read of VW magazines or groups such as this makes it clear that disinformation is the order of the day, with bad advice out-weighing the good by a wide margin. A nice example of this is the popularity of pulleys smaller than stock diameter, guaranteeing the engine will run hot.

Operating the vehicle in a conservative fashion with careful attention to the ambient temperature is another critical factor that is largely ignored. Having nothing to freeze, air cooled engines do best in cold climates rather than hot but the fact Berlin is as far north as Winnipeg escapes the notice of VW owners in Florida, southern California and Texas (Brownsville is farther south than Cairo... but don't tell anyone :-)

After-market heads having greater thickness in the areas most prone to cracking offer some relief but only at the cost of poorer cooling, since the thicker casting can only be achieved by reducing the depth of the cooling fins. Reducing their depth reduces their area which guarantees the heads will run hotter. Enormously popular, of course :-)


So what to do?

If you aren't willing to become a pretty good VW mechanic, the wiser option is to buy a Toyota and get on with your life, because sure as babies shit green, if you are forced to rely upon someone else to do your maintenance and repairs, you're gonna get screwed.

But having said that, the basic cure for a cracked head is to replace it. (I've described how to repair a cracked head in an article posted... somewhere... some years ago. Bottom line is that if you aren't a pretty good TIG weldor with a shop full of tools and a life full of experience, forget it.) You need to make sure your replacement head is IDENTICAL to the original otherwise you'll upset your compression ratio and valve train geometry. But when an identical head is on-hand, the task is a simple R & R chore -- remove & replace.

There are a few details that must be taken into account, such as the fact it's always best to replace both heads rather than one at a time, or understand the need to slack off the stays on the good head so that both will see equal tension once you've finished torquing-up. Plus the VITAL need to drop the engine and re-torque the heads after about six hours of run-time, this to accommodate the malleable nature of cast aluminum and its need to conform to the cylinder's sealing surface. But on the whole, swapping heads is a relatively simple task.

Yeah, I know -- I could hear that from here :-) So lemme restate that...

Swapping heads is a relatively simple task when you understand the need and reason for doing so. In fact, in the proper management of your vehicle's maintenance you won't even wait for the head to crack, you will keep a spare pair of heads on-hand and REPLACE them as a matter of course every time you do the clutch... when you will also re-set the end-float, replace the main oil seal and so forth. That event will take thirty to forty thousand miles of driving, depending upon your locale. Yes, the clutch still works but here again, you are performing PREVENTATIVE maintenance -- scheduling your time in order to deal with problems BEFORE they occur. When you plan your maintenance in this fashion you will see there are logical sets of tasks that may be performed at the same time, such as swapping-out those crack-prone DP heads for a spare set. Now you have several years of trouble-free driving during which you can overhaul the old heads or obtain replacements.


To return to your original question, the most characteristic sound of a cracked head is the jingle of a cash register :-)

-Bob Hoover