The Stovebolt Site Forums The Shop Area Engine Shop Rear Main Seal

If you find one of these seals made by Fel-Pro my experience is it will not be hard and brittle no matter the age. I have some I've saved for 50+ years by the way and they are still just like new. I have been told GM did not make this seal but outsourced it. Around 1999~2000 I spoke with one of the Fel-Pro engineers who had been on the team who had developed that seal. He said while it was commonly referred to as a neoprene seal it wasn't exactly that. He said it was a proprietary blend of compounds he couldn't discuss. We also talked about the practice of offsetting the two pieces by rotating one a bit in the channel. He paused and said something like "well as long as you don't exceed the offset about 1/16" it probably won't hurt." He added "I wish I could say that's a good idea but we don't suggest it because our research showed it was not necessary and we could not control how much people would rotate the seals." He said if you just follow the instructions on the seal package all will be okay. He added that seal had been designed to work on a nice smooth surface within a specific crank to bearing clearance and .001" would be ideal. I asked about a worn crank with a clearance of say .003". He said "you're on the outer edge of good practice there unless you are using a one-piece spring lip seal (which was never made for the 235)". He ended by saying he was less than a month away from retirement. Nice guy and very interesting. I was impressed he took as much time to discuss it as he did.

I would suggest doing this: install the right Speedi-Sleeve using red Loctite where your crank meets the seal. If you just have your crank polished where it meets the seal, then add some shims behind the seal piece until you can not draw a 1 inch piece of common copy machine paper out without it tearing. Unless you did the machining, you don't know how much metal was removed. You can use foil HVAC tape cut into long thin pieces if you need shims. Each piece should add about .004". Also no crank that was mated with a rope seal I've seen in the last 60+ years has had a smooth enough surface for the rubber type seal. Any roughness will wear that seal out tout de suite. I have images of some I've saved but I'm not where I can access them now. Dealerships commonly installed this seal when they could because they could do it without removing the engine. Often they failed pretty soon.

As for Best Gaskets, I don't know what they're made of and I think they're absurdly overpriced. Personally...I'd look for a genuine unused Fel-Pro seal if it were me. Get one made by the company who engineered them and not by somebody who just copied them. Good luck!