C2 Rear Main Bearing Cap

[Mark P.]

My rear main bearing cap for my 65 small block has one edge of the pan seal retaining lip busted off. Here's the question: If I replace it, can I use the same bearing that is currently in it, or will I need to get it plastigagued because tolerances may have changed and then get a new bearing based on the new measurements?

I guess the other question is, how many of the main bearing caps are interchangeable or will I need to get one from an original engine?

Thanks for the input!

Mark

[Clem Z.]

Re: C2 Rear Main Bearing Cap

you can not just change caps without line boring. find a good welder and have him build up the area needed with with braze or weld. i have had this done with no problems

[Michael H.]

Re: C2 Rear Main Bearing Cap

Mark,

The rear main brng cap on a small block would be basically interchangeable with any cap from 1959 through 1967. The problem is, this part is typically not a "bolt on and go" item. Typically, the block/caps would require reboaring or honing to establish a true round/straight crankshaft bore that is the correct dimension. The original caps were assembled and torqued in place on the block in production before the main bores were cut. Since the size, roundness and alignment are critical dimensions, any misalignment or variation in size would certainly have a drastic effect on bearing life. I know of two people that managed to pull this off years ago without align boring but that's certainly the exception, not the rule.

[Joe L.]

Re: C2 Rear Main Bearing Cap

Mark------

I'm afraid that you have bigger problems than you think. If a main bearing cap on an engine is replaced, the engine must be line-bored or line-honed to "fit" the bearing cap to the engine. This means that the engine will have to be removed, torn down, a new bearing cap fitted to the engine, and the block then line-bored or line honed.

Reusing the bearing may be possible, but I'd never do it. Main bearings are not all that expensive; I'd just use a new one, properly sized for the engine. That's the very least of your problems, though.

The rear main bearing cap for your engine is long-since GM-discontinued. However, you can use a rear cap from any 1959-66 small block engine. These will be identified by the casting number 3746464.

[Philip C.]

Re: C2 Rear Main Bearing Cap

Hi Mark thats a NO and a NO How bad is it broken? you can have it repaired. main caps dont interchange! Your in deep do do to replace the cap your going to have to have it line honed. send a photo I have some old main caps for parts.Sorry Phil 8063

[Michael H.]

Re: C2 Rear Main Bearing Cap

I think your idea is best Clem. That can be done without removing the engine/crankshaft. Low heat shouldn't affect the cap.

[Mark Milner]

Federal Mogul says no.

A bearing has crush built into the dimensions for when you torque the cap down. So F-M recommends never reusing a bearing once a cap is torqued.

Obviously, that makes it hard to use Plastiguage to check clearances, but I think they mean once it has been run and heated.

I never remove and use Plastigauge on an old engine and then torgue the caps back on, if I plan on running the engine. Once removed, I put in new bearings.

I cannot count the number of people I have known of who have removed and retorqued caps with old bearings only to have bearings spin.

Of course, you have already seen the other problem you have, having to align bore the crankshaft mains.

[Clem Z.]

i have freshened race engines with new rings

a never replaced the rod bearing inserts and never had a problem BUT i have know people who have replaced the rod bearing inserts with out reconditioning the rods and had then spin a bearing the next time out. as long as you put the insert back where it came out of no problem. i checked all my bearing clearances by assembling the mains and rod with bearings,using a dial bore gauge to check the inside diameter and then mic the outside diameter of the crank journals and subtract the difference to get the clearance. of course i had to take them apart and reassemble them in their proper place,this is standard engine building process.

[Mark Milner]

Re: i have freshened race engines with new rings

According to F-M, you have been lucky.

I've never understood why you couldn't, but according to F-M, bearings are slightly too long, so that the ends are pressed against each other. This holds them in compression and keeps them in place. Once the bearing has heated and used, the bearing loses any springiness and will not retain pressure if removed.

I guess it is quite possible you hadn't heat cycled the bearings enough to remove the springiness from them.

As to new inserts spinning in rods not reconditioned, I would think the rods have to have a lot of wear or extreme use so that they had eggshaped slightly like they do over time. But that is normal to any rebuild.

I'm talking more like the guy who puts it together, drives a few race weekends, then sticks new bearings in along with new rings.

[Joe L.]

Re: i have freshened race engines with new rings

Mark------

Bearing caps and bearings are routinely removed to check bearing clearance. Sometimes, this is done several times. Sometimes, bearings which turn out to have the wrong clearance are removed and placed back in boxes and another one installed in the engine.

GM makes no requirement, whatsoever, that bearings be replaced when caps are removed. In fact, GM overhaul manuals state that the bearings can be reused as long as the clearances check out ok. Of course, most folks would not re-use bearings at rebuild time whether, or not, clearances check out ok. But, GM says that you can.

Also, rear main bearing caps are often removed for the purpose of replacing the rear main seal and rarely is the rear main beaing replaced at the same time.

Running the engine up to temperature has absolutely nothing to do with "setting" the bearings or reducing their "springiness". So, if one cannot reuse a bearing once the cap has been installed and torqued, then one cannot "re-use" the bearing after checking the bearing clearance with Plastigauge. That would make checking bearing clearance self-defeating.

[Mark P.]

Re: C2 Rear Main Bearing Cap

The portion that is broken off is about 1 inch off of the driver's side on the edge, so I suspect it could be repaired. I would much rather do that than replace the cap and then have to line bore! I'll give that a try and go with new bearings. I knew it was going to be more trouble than a simple bolt on and drive away. By the way, I didn't drop it, if that's what someone was thinking..... I found it that way when I went to replace the pan gasket. Honest! Thanks for the advice, guys.

Mark

[Clem Z.]

Re: i have freshened race engines with new rings

the race engines i built would run maybe 500 laps i would replace the rings but if the bearing unserts looked good back in they went. never lost a bearing in any engine i ever built and some of these were 8000 RPM drag engines.i never replace a bearing insert without reconditioning the the rod. one thing that makes for good bearing life is to align the rod and piston in a fixture to make sure the rod big end is perpendiular to the piston so the bearing runs flat on the crank. the bearings are held in place by "crush" which is what you discribe each half is slightly longer the a true half so they are like a "press fit" in the rod but that does not change with use. sometime a insert will loose it "snap in" fit but all you need to do is spread it with your finger till it snaps back into the rod. all the rods i used even new ones i would have the big end reconditioned to the "small" end of the spec diameter to get the max "crush" on the insert.

[Mark #28455]

is it just the lip or the seal surface?

Was it a running engine? Was it leaking?

If it's just the lip that holds the seal in place and it wasn't leaking, I would consider using a 1 piece pan seal and forget about it. You could test fire the engine without any accessories installed to verify it's not leaking if you want to. If it WAS leaking, then you have to go further.
Mark

[Mark P.]

Re: is it just the lip or the seal surface?

It was running great, just leaking like a sieve. When I pulled off the clutch bellhousing front cover, I saw that the pan seal had shifted because the piece of the retaining lip was missing. It sounds like I may be able to get by without replacing the bearing when I repair the cap. The bearing surface does look good, so I think I'll take Clem's advice and try to use the current bearing.

Thanks to all for the input.

Mark

[Mark Milner]

While both you and Clem are probably

right, Federal Mogul is the one who made the statement of not reusing used bearings in one of their papers. Unfortunately, I don't have a copy of that.

This could be looked at in several ways:

1. F-M makes more money with new bearings each time.
2. GM is completely correct for their intended life of a vehicle, which is much shorter than we intend.
3. I've known the few unlucky guys out of the 1000.
4. You guys were the lucky ones out of the 1000.

Anyway, since I was replacing bearings for around $50 for the full set, it was cheap insurance as far as I was concerned. With a tank of gas for more than that today, it is cheap to replace them.

Now if I had a lot of miles on a rod, I wouldn't replace a bearing either without reconditioning the rod.