The Stovebolt Site Forums The Shop Area Engine Shop GMC 270 Rear Main Seal Mod

I'm looking into modifying a GMC 270 rear main seal to accept a rubber seal rather than a rope seal.
There doesn't seem to be anyone that makes a seal for the 270 but if I can find something I can adapt to it I might give it a try.

Part of my motivation is that the seal surfaces on the two cranks I have are fairly pitted. Stock crank seal journal diameter is 2 7/8" but I could pretty easily grind it down to 2 3/4" dia. or anything in between.
I can bore out the back of the block and main cap to accept a seal. It may require me to make a bushing to hold the seal, depending on what seal I can find.

I see BestGasket makes seals for GMC/Chevy 235 & 261's. https://bestgasket.com/rear-main-seals/ Can someone tell me the diameters of the crank seal surfaces on those engines? From what I can find they're different than the rear main journal. They look to be designed to fit in the existing rope seal groove so may not work for what I'm doing.

I did find "split" one-piece seals from Garlock like this: https://www.garlock.com/catalog/products/bearing-protection/oil-seals/split-seals/24610-0648
They make tons of different sizes and materials with either single lip or with dust seal lip. The hard part might be in getting the seal onto the crank and past the oil slinger. It would likely have to be put on the crank before it was installed.

I'd probably just put in a rope seal if the cranks weren't pitted. (one crank has already had the seal surface ground down about .009") Not sure how much smaller it could go and get a rope seal to work.
What do you think? Is it a waste of time or do you think one of the Garlock seals would work?

I just measured the seal surface on a 270 GMC crankshaft. It's 2 7/8" (2.875) in diameter, and 3/4" wide. I'd suggest finding a 2 piece crankshaft seal that's close to the right diameter (maybe a big block Chevy V8 or something from MOPAR), grind the GMC crankshaft seal surface to that diameter, and make a 2 piece seal holder that bolts onto the back of the block and the main bearing cap. There's no point in reinventing the wheel when a simple resize of the crankshaft will give you a smooth surface, and a seal that's already been engineered to work with an engine.
Jerry

How about if you just buy and install a 99287 speedi-sleeve"? It is made for a shaft of 2.8752" and will add just a wee bit to the overall diameter when installed. Install it using red Loctite. This will give you the equivalent of a brand new sealing surface back there and will provide some good alternatives. You can usually find those on Amazon for about $30.

Can't do a speedy sleave because of a large flange on the back end of the crankshaft where the flywheel mounts.

I've built 2 of these engines both 270s and one has nearly 40K Miles and doesn't leak a drop. this is not to say it doesn't leak other places .