Minor corrections/additions to a wonderful post:
Quote:
Originally Posted by Red Coupe
If the rear main seal groove was not concentric around the center line of the crank, the two readings will be different.
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The two readings could be identical and the seal "could" still not be concentric. This could happen if the seal groove is displaced too high or too low rather than too far to the left or right.
Better measure top 'n' bottom, too. You'd have four measurement points that better all be about the same.
Quote:
Originally Posted by Red Coupe
Apparently when the factory installed the original asbestos style rear main seals back in the 60s and 70s this was not a problem. With the newer style seals, the ability to compensate for variations in machine work isn’t as good, and this problem can lead to a nasty leak. My machinist says he has only seen this error on 455 blocks. Nonetheless, once burned, twice shy, so he always checks when working on a Pontiac motor.
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It's not a matter of asbestos vs. modern seals. It's a matter of "Rope" seals which may or may not be asbestos vs. lip seals which can be neoprene, Viton, or some other "rubber" material. A lip seal has a very limited range of allowable misalignment with the shaft, AND they need a fairly smooth surface for the lip to ride against.
Of course, you aren't going to buy an asbestos seal any more; I think the modern rope seals are a fiberglass material. And they don't seem to work as well as the genuine asbestos material--but--the EPA and OSHA frown on asbestos. Even so, if you have a block/cap that has the seal groove not concentric with the crank centerline--rope seals are your best friend.
I don't like rope seals except for agricultural or industrial use where the seal has to deal with abrasives--such as a pump used on "contaminated" liquids. (So I avoid using them on automotive engines where there shouldn't be any abrasive in the oil) Sometimes there's no choice, though--either the seal groove isn't concentric (some Pontiacs and perhaps others) or the crankshaft isn't smooth enough for a lip seal to rub against and live.