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

I'm working on rebuilding a 270 GMC and the seal surface on the crank is pretty pitted and has been already been ground undersize a little. I'm considering modifying the block to accept a rubber rear main seal.

I found a Garlock split seal that is a size that I can counterbore the back of the block to accept the seal. (see pics)

Boring out the block should be fairly easy. I can set the block on my mill sitting on the top deck and be able to bore it using a right angel head on the machine. The installation instructions call for a seal retainer to be installed which I can make and retain with some small flat head screws. With the one-piece design of the split seal it will require me to put the seal on the crank before setting it in the block but I think that will work ok.

Basically I was wanting to bounce the idea off folks in this forum to get your reaction to the idea. Do you consider it a workable idea or would you say no way?

That might work. Do the seal ends need to be glued together? Also it looks like your seal retainer would need to be a 2 piece affair, secured to the block and the main cap separately. That flywheel flange on the crank sure makes this harder than putting a full circle seal on a Chevy. I did that on my 216 when I put the 235 crank in it.

Yes, the retainer will have to be two piece. The instruction only say to orient the split in the seal towards the top but no mention of sealing the ends.
It would definitely be much nicer if it didn't have the flywheel flange.

I'm working on a similar idea. The 2 piece seal for a Chrysler 440 engine is pretty close to the diameter of the GMC seal surface. I'm considering milling a flat area on the back of the GMC block and rear main bearing cap, and making a seal holder from aluminum that will occupy the same area as the rope seal does now, then grind the crankshaft to the correct diameter for the seal. The seal holder would attach with flat head screws.
Jerry

I purchased 2 Garlock seals, not being sure which I'd end up using.
1) #25003-0806 3.560 OD x 2.812 ID x .375 thick
2) #25003-1039 3.740 OD x 2.835 ID x .375 thick

Looks like either one will work. The advantage of these seals is that they don't require a seal adapter. The OD size is larger than the original seal groove so the original groove can be machined away and the seal fit directly in the block. It will require seal retainers since the seal is open to the rear and the seal will have to be on the crank when it's set in the block.
It appears the Chrysler 440 crank has a 2.81 dia. seal surface on the crank. If I use the seal with the 2.812 ID, and it doesn't seal properly, I could make a seal adapter and use the Chrysler seal. It would require an engine tear-down though.

Just occurred to me that my seal retainers will have to be screwed fast after the crank is in place, leaving no room for an allen wrench between the block and flywheel flange.
I'll have to make sure the screws for the retainers line up with the flywheel bolt holes well enough to get the screws in through the flywheel flange.

I was wondering about being able to put the seal retainers in place before the crank is set in place and the cap installed, but you'd have to be able to be sure the seal settled into place between the retainer and recess in the block/cap. But accessing the screws thru the bolt holes in the flywheel flange should work well. Just a bit fiddly. Maybe loosely install the seals and screws before the crank, then tighten the screws thru the flywheel bolt holes.

The GMC seal surface on the 270 crankshaft I just measured is 2.875", so there's just enough diameter difference on the MOPAR 2 piece seal to grind the crankshaft to fit. I think the 2 piece approach would be a simpler fix.
Jerry