GMCs and Chevys have one thing in common- - - - -rear oil seal leaks. Both engines "mark their territory" on a regular basis. Since a Chevy crankshaft has a straight rear flange, it's a fairly simple machining operation to counterbore the back of the block for a full circle lip seal- - - - -a Timken #417210 or equivalent. If the seal surface is pitted, use a SKF #99387 speedy sleeve or the equivalent to smooth up the sealing surface, or have your machine shop polish the shaft to smooth it up. The lip seal will handle up to a +/- .015" diameter shaft without leaking.
Not so for the GMC- - - - -the flywheel mounting flange is much larger than the seal surface, so GMC owners are stuck with using a 2 piece seal. Rope seals might have been OK 50 years ago, but there should be a better option these days. Well- - - - -actually, there might be! A 2 piece neoprene lip seal for a big block Chrysler V8 is made to fit a crankshaft sealing surface that is 2.815" in diameter- - - - -compared to the GMC seal diameter of 2.875". Both sealing surfaces are about the same width. It would be possible to have a machine shop grind the GMC crankshaft to use the smaller diameter seal- - - -.060"- - - -just under 1/16" difference. Then it would be necessary to flycut the back of the GMC block and rear main bearing cap for a 2 piece seal holder, attached with machine screws and a thin gasket. I'm in the process of figuring out how to do that, with a GMC 248 block that was donated to me a couple of years ago. 228, 270, and 302 engines should be similar, I believe. Stay tuned for updates!
Jerry
