Tuesday, November 21, 2006

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.

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