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
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
You are correct about the 228, 248. 270 and 302 all using the same rear seal. That family also includes 236 and 256 engines, but those are very rare.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
I helped rebuild a 256 several years ago- - - - -its original application was in a Canadian military vehicle. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
The two piece neoprene rubber seal on a Cadillac V8 is too big.. but also has no special holder or need of a screw made into the block. There's just a deep groove into which the rubber seal fits, and the flat mating surfaces just mash together when you put the bearing cap on. So I wonder if there's a similar type of seal available for the smaller diameter? Or, since the Cadi seal literally slides in with a little plastic spoon behind it to keep the rubber back side from tearing when you push it into the upper half with crank still in place, could it just be cut down to the right length? The experiment would only cost about $12 to do. Edit: $13.98 on partsgeek.com.. further edit, some overhaul kits for the 425-472-500cid Cadillac V8 come with a rope seal option. Which of course you shouldn't use because this is available. Point being, the block oil seal grooves are broadly the same, so I wonder if the seal could work as well.
It would depend on the width and depth of the neoprene seal- - - - -it's not very likely that it would be a direct replacement for the rope seal, but at that price it's worth a try. If the seal can be made to fit the groove in the main bearing cap correctly, the same dimensions would apply to the block groove. If the groove is slightly too deep a piece of brass shim stock could be added to the bottom of the groove with heat resistant epoxy cement. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
