The Stovebolt Site Forums The Shop Area Engine Shop rear main bearing cap

Ok, I a getting ready to put the oil pan back on. I cleaned the oil pump and am cleaning up the old gasket etc. Then I notice the (what I assume is the) rear main bearing cap has several huge cracks in it. It looks like they are near the bolts like someone overtightened them or something. I remember the PO told me they had a lot of trouble getting the seal to stop leaking there.

Now, these cracks can't be good can they? What do I do? Can I replace just the cap and the lower part of the seal? Is the bearing involved too? Are there different size bearings? Will this ever end?

You can't replace the cap without line boring the main bearing. I can't imagine that any cracks in a main cap are good, so you may have to spend some $$#$ to get this one right.

Well, I went under there again. The cap seems cracked on both sides but on the outside of the bolts. So, even if it were to fail the bolts would hold the center piece of the cap and the crank in place.

Did you pull the main cap to check it out close up and personal? No, you're not supposed to replace just the cap, but you will in this case.What have you got to lose? The engine is already junk.Pull the spark plugs,turn the engine over, get a feel for the rotation.Find another cap,put the bearing shell in it, a main seal.bolt it down.Turn the engine over, feels ok?Start it up, got normal oil pressure? no knocking?
What caused this main cap to crack?If it was a race engine, a lot of horsepower and detonation cracks mains.Now is this a future sign of trouble for all of us?

This is what killed my '54 235. The front main cap was cracked on mine, a vertical crack running down the outside of the bolt hole. My crack runs from below the bolt head down to the cap parting line. Since the closest line-bore shop is in Georgia, my engine is junk.

Listen to the voice of experience: If you find a cracked main cap, your motor is junk. Doing anything but tossing that block in the closest smelter is pure folly.

I know you're in the middle of changing something or other, but a cracked cap is a TKO. You're wasting your time. How do you think I felt after installing 300 dollars worth of clutches, gaskets, tranny bearings, etc just to find out my motor was junk???

Bring the cap to a machinist or post pics here to verify that it IS cracked.

Alternatives? If you're easy on the engine, it may last for awhile.

So find another engine, and run this one 'till it flies apart.

I think this is caused by vibration and metal fatigue. The youngest 235 was built 40 years ago, and the cast iron in these engines is starting to crystalize and fail. The place where this failure will show up first is around the main cap bolt holes.

Chip

These storys are becoming like some "bad" nightmare. Next thing I'll hear is that you are all becoming "rodders" dumping in new 350's or such stuff. Tony M, where in Calif. do you live? I have a running 53-54 235 that you can have for the getting. I'm sorry to hear about that main cap. Don't run the engine just keep what parts you can for another engine. I also have a 55 235 but it needs bored which will mean new pistons (extra money). But it is stock so should only need to go 20 over. Your welcome to either one.

Well I decided to take the cap off and inspect it further. But man those bolts are on tight. I got the front two loose but the back two were pretty immovable. As I layed there contemplating my belly button I took my fingernail and scratched at one of the cracks. It came off. I fiddled with it some more and it seems it may have been some hardened carbon. It is dang dark under there. I will check again with a good light but it seems this may have been a false alarm.

Glad you lucked out!

We're gonna take up a collection and buy you a wire brush and a flashlight. :p

Chip

If you can, use an impact wrench to remove those bolts. Then throw them away, and get some new ones, it sounds like they have been overtorqued and stretched. That means that they will not torque properly, and will possibly come loose, maybe even be cracked.