Flywheel or "Rear Main" Seal Story

Any name-brand seal is as good as any other as long as there is proper drainage!

The "

rear main seal

" on VW flat-four engines (all of them) does not actually seal against the crankshaft. Instead, the flywheel bolts to the back of the crankshaft with an O-ring in between, and the lip seal (pictured) seals against a polished surface on the flywheel. The flywheel seal on all 1972-79 Buses and all years of the Vanagon aircooled and watercooled flat four engines (not diesel, which is just a Rabbit/Jetta diesel engine) all have the same outside diameter (OD) and inside diameter (ID). The difference between the air seal on the aircooled engine and the water boxer is the thickness (front to back, or width). The former was about 10mm and the later was about 12mm. The reason for this difference is unknown, and both seem to be interchangeable without any issue. 

For many years about 9 out of 10 oil leak issues we encountered on our rebuilt engines was flywheel seal leak. This has been a reoccurring problem since forever. There are probably some of our engine customers reading this who know what I am talking about. We have gone out of our way to make sure every flywheel we install not only gets the thrust surface ground perfectly smooth and square, but everyone gets the sealing surface polished to practically a mirror finish. Even still, one in twenty of our engines, or so, would LEAK! I just figured there must be something up with the seal. And indeed, we found differences between various brands and part numbers, and tried all of them—but the problem persisted. Sometimes the seal would completely blow out of the engine block—and that was a very valuable clue. This got us thinking that perhaps the problem all along was a build-up of pressure between the engine block and the seal. Here is a photo of the passageways that VW machined into the block parting line for oil to return back to the crankcase from the area between the seal and crankshaft:

Our theory was that maybe these seals might be getting clogged over time, or perhaps some excessive sealant could be getting in there during assembly. To test our theory, we began drilling a new hole from this same area into the crankcase using one of our seal installation tools modified to include a drill guide:

The result is a hole safely drilled through the block:

And, voila! Our flywheel seal leak issue all but disappeared! And now, all of our flywheel installation tools include this added feature so anyone doing this job can safely add an oil return hole and install a new seal for the last time!

Yeah: GoWesty’s got your back, and the back of your engine!