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Apr 26th, 2026 | | AD Chevy Trucks  Over 6,000 pictures Brad Allen has an awesome collection of Chevrolet factory pictures that he has set up from film strips. Check them out!
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| | | | Joined: Feb 2004 Posts: 29,740 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 29,740 | 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!
| | | | | Joined: Jan 2022 Posts: 2,435 'Bolter | | 'Bolter Joined: Jan 2022 Posts: 2,435 | 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.
| | | | | Joined: Feb 2004 Posts: 29,740 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 29,740 | 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!
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