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Flywheel or "Rear Main" Seal Story

All seals are equal, but some seals are more equal than others.

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 72-79 Buses and all years of the Vanagon air and water cooled flat four engines (not diesel, which is just a Rabbit/Jetta diesel engine) all have the same outside diameter (OD) and inside diameter (ID). All the seals we measured had the same OD and ID. However, the thickness (front to back, or width) were different on two of them, and on one (the dealer part, which was, of course, the most expensive by a factor of about 10) had a substantially larger diameter spring wire diameter (22%). Why are we talking about this?

Aftermarket seal on the left, OEM Volkswagen seal on the right.

Well, one of the most common problems we have to address on the waterboxer engines we build are flywheel seal leaks.  This has been a reoccurring problem since forever.  There are probably some of our engine customers reading this that 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 me something up with the seal.

So, I decided to buy one of every seal we could get our hands on.  In the process of my investigation, I heard all sorts of theories, like "You gotta use this super gee-whiz goop" or "You gotta put it in with a golden hammer" or "You gotta use the factory VW, OEM seal only available at the dealer for ten time the cost."  I had no trouble believing everything I was told, except the last one.  I KNEW that was BS...

Well, good thing I did not bet any money. Sure as $#%@, the damn VW dealer-only seal is in fact different. It was actually one of our shop techs that pointed out the difference to me. I wasn't sure if the guy was BSing me or not, so I had to go through this exercise. I should've just listened to him in the first place! 

Anyway, what we found was that the common seal that comes in ALL the gasket kits are different in two ways. The first is that they are narrower (width) front to back. So, when you install them in a water-boxer, you can put them in too far, not far enough, or catty-wompus (that's technical jargon for axially misaligned). We found one seal that was the correct thickness, a Victor-Reinz part that was all orange, and sold separately—in its own box and everything. Interestingly, it is NOT the same seal that is included in the Victor-Reinz full engine gasket set. Go figure. So we thought, viola! We don't have to buy the stupid expensive seal at VW. WRONG. What our tech correctly pointed out is the second way that the OE VW seal is different: It comes designed with a considerably heavier spring. None of the other seals have a spring nearly as heavy. Kudos to our shop tech, right? 

What is even more interesting about this VW dealer-only seal is that it is all black and it is made in Brazil (SABO brand). These two traits are typically what we refer to in the industry as... well... CRAP. But, you can't argue with success… 

After much back and forth, lawyers, guns and money—we were able to procure the exact same seal the VW dealers sell, but directly from Brazil—instead of Germany.

This is the same seal VW was selling – back when VW still sold them. Everything was going grandly for quite some time, until… the leaks started coming back and with a vengeance!

As it turns out, the quality of the Brazilian-made seal ended up deteriorating over time. A close examination of the seals we were getting by 2020 showed signs that the mold they were being made in was just plumb wore out. You could see left over flashing along the parting line on some of them, RIGHT where the sealing is supposed to happen. So, it was back to the drawing board.

Fast forward to 2021 and we now offer three seal options on our site: The remaining, good SABO seals (while supplies last), the common German brands Victor Reinz or Elring seals which are commonly available. We are in the process of determining which of these latter two seal brands is the best. This will take time, over hundreds of engines before we know if one is really any better statistically than the other. So far, the jury is still out…  

The other problem of both of these seals being narrower we have figured out. They both require great care to get them in straight, every time. To do that, a special tool is required. To get that, a special and large amount of money used to be required. So, we rolled up our sleeves and made an economically-priced tool for the job!

Installation note for off-road environments: The photo below is of a flywheel removed from an off-road race buggy a friend of ours owns. He is running our 2700cc Waterboxer and GoWesty EFI, and recently had a rear main seal fail. It has approximately 50 total hours, some of which were in very severe conditions including being fully submersed in fine desert silt. The sealing surface of this flywheel was perfectly smooth when it was installed. As you can see, there is now a very deep groove in the flywheel which resulted in a very significant oil leak. All engines installed in Syncros at the factory were sealed to the transaxle bell housing with a thin layer of silicone sealant to help keep debris out of the bell housing specifically to prevent this from happening. In addition to that, the access hole on the engine block located around 10 o’clock, which normally has a plastic cap on it, was fitted with a rubber cap and breather hose on Syncros. So, installation tip: if you plan to run your Bus or Vanagon in extreme conditions, or even if you are not, it is a good idea to seal up the bellhousing/engine block assembly as good as you can, with just a small opening so it can breathe.

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