The 312 crank is larger in diameter

The crankshaft seal surface for the 292 and smaller Y engines is 2.625” while the crankshaft seal surface for the 312 engines is 2.750”.  Hence the need for two different neoprene seals for the Ford Y-Block.  The Fel-Pro seal is specifically for the 239/292 engines and will not work in the 312 engines.  Installing the 292 neoprene seal in the 312 engine simply redefines how bad a rear seal can leak in a 312.

My first experience with the Chrysler/Mopar seal was with a customers’ 312 engine many years ago that came into the shop and was leaking profusely from the rear seal.  That MoPar seal had been hand cut to fit the 312 and because of the rough cut, simply leaked.  Working on the rear main seal by simply removing the oil pan with the engine still in the chassis, the seal was replaced with a correctly trimmed Mopar neoprene seal and that fixed the leak.

Prior to Best Gasket coming out with a neoprene seal for the 312 engines, I was using the Chrysler seals that were modified and sold through FordCraft (Don Hyde) in Dallas.  Those worked well and I had no issues with using them.  It was common knowledge at the time that a number of 312’s did have irregularities in both the seal groove width and depths which would be problematic when installing a neoprene seal.  When those issues came to the forefront, rope seals were the fix for that.

When Best Gasket initially came out with a specific seal for the 312, it was orange, but those seals used a softer material and would fail in less than 500 miles.  BstGskt then switched to using the same black material that is used in the 239/256/272/292 seals at which point the wear problem with the earlier orange seals went away.  I have used the BstGskt seals for the 312’s with great success but I am aware of some 312’s having a wider/deeper seal groove which could end up being a problem with the seals not doing their job.  When I do find a problem with the rear main seals, more often than not the issue is with the side seals not sealing adequately at the seal retainer to block interface.  A dollop of RTV in the side seal groove before installing the side seals seems to fix that problem.

Here’s the link to a discussion a few years back where I went into detail on a rear main seal leakage issue that came into the shop.  Although this was a 292 engine, some of the same pitfalls you will have with a 312 engine was taking place here.
http://forums.y-blocksforever.com/FindPost145797.aspx

Lorena, Texas (South of Waco)

Thank you Ted,  I guess I should preface this with saying that the Best Gasket seals did not work in this particular engine.  We found that the rope seal was impossible (for us) to put in the engine.  We were taking an engine to Tim McMaster for overhaul, and spoke to him.  He was not a fan of Best Gaskets and said that he exclusively uses the Chrysler seal on the engine he builds.  After hearing that, we ordered the Chrysler seal, trimmed the flaps and also trimmed the small extensions on the exterior of the seal and installed it.  Bingo!  We started the engine and ran it for 30 minutes with no leaks!   If the Best Gasket seal works for you, then use it,  After 3 failures and rewatching the video to make sure that everything was done according to the instructions, we are probably going to continue to use the Chrysler seal in the future.  Other's experiences may be different.