I would like to share some information about the stock case. The first thing practically every shop will tell you is to line bore your case if it is worn. Have you ever asked, "Is this rebuilding the case?" Not so, according to a document on the applications and effects of magnesium when used for engine and transaxle housings that are heated and cooled many times. The document was written by the Quality Control Department of a major German car builder. It disclosed that the engine cases made with the AS41 material are designed as non-rebuildable items. In other words, when it needs to be reconditioned, replace it with a brand-new case. Naturally, this will depend on how bad and how much more use you think you can get for the money you will spend patching it up. Remember, you are only patching it up to run a little longer, not reconditioning it.
Why does the case pound out and get bad in the first place? Why does it have over-heating and low oil pressure after it is rebuilt? What are the facts about all this?
AS41 magnesium is a very lightweight material. When used in ideal circumstances, it will give 100,000, maybe even 150,000 miles of trouble-free service in the VW engine when never overheated or ran over 4400 RPM. When I read available documentation about the AS41 and AS21 material used in the VW cases, I was shocked when I saw how weak the AS41 is when subjected to conditions beyond the normal VW temperatures and stress. This, however, substantiated many evaluations I made from my own years of research and development and answered some questions previously unanswered regarding the memory, rigidity, resistance to elongation and strength which was reduced drastically.
The higher the temperature, the faster the case is reduced to junk! A new material called AS21 was available, however, it required slower, more expensive production procedures and was not introduced until the FI engines in 1975. The Volkswagen factory of Brazil has informed us that the AS21 case containing 2% aluminum and 1% of silicon have not been available since 10/25/95. They have been replaced with the AS41 case. AS21 is stronger and resists problems of the AS41. The AS21 markings can be found at the side of the case and often in large letters in the bell-housing area.
Most engine builders have noticed these cases show a considerable decrease in pounding or distortion as compared with the AS41. When any of the cases are overheated (AS41 material being the worst) they are stretched, pushed out of shape and the center main bearing area starts to wear. The materials are subject to stress beyond their design limits. This overheating is more apt to happen to the smog controlled engines (1968-on) that are run very lean, which causes overheated heads that come loose or head studs to pull out. The more times this happens the worse it gets. This is irreversible damage. It soon pounds out the center main saddle enough so oil leaks between the case and the main bearing.
This loss robs the engine bearings of needed oil. More heat is produced due to less lubrication and more damage occurs. Soon the number one bearing starts to do the same thing. By now the case has lost most of its resistance to elongation and the center main saddle is pounded so badly that the cam tunnel has moved over and is wearing the center cam bearing. A non-counterweighted crank only aggravates this condition especially when the engine is often turned to its upper limits. Even if a properly counterweighted crank is used under the conditions I spoke of, some of this damage may still occur. Some of this stress can be dealt with by stress-relieving the case 3 or 4 times, removing the case studs, machining both halves flat, re-install the studs, torque everything, and remachining every surface to the proper size. This is still not a guarantee of being 100% like new and at today's labor prices it is out of the question.
A new case is far cheaper and will assure 100% life ahead. That's one of the reasons Volkswagen cast new cases for their factory rebuilt engines from the mid 60's and on.
I tested 100 line bored cases in stock 1600cc engines. Over 80% had low oil pressure when normal temperature was reached. This often occurred in less than 5,000 miles. Most of them never performed well and got poor mileage. FAT Performance line bored the case on one of their 1600cc off-road race engines. The dyno revealed that it was down 17 HP from previous tests.
Another engine that made the correct HP was disassembled and every part, piece by piece (except the case) was swapped, still down 17 HP. I was called in as a consultant. My only comment, "Replace the case." This advice was not taken at first. In fact, a newly designed head was tried first. Better, but still not up to normal. Finally, Ron went for the new case. "Voila!" a magical 19 HP appeared. Ron said, he has heard me saying this for years and never paid much attention, but now had the actual experience of what I have been seeing for years.
If nothing else, use your common sense. When you use up 75, 80, 90% of the life of the part why put it back into service and ask it to take more stress and strain than it did when it was new? Don't spend a bundle for machine work on an old worn out case and expect it to give the service of a new one? It is your gamble. What is it worth to you to know it is good for 100% of its life rather than 10% or 20%?