Rear Main Cap Opinons

kippmw

kippmw

Member
I've noticed recently a low knock on cold starts that would go away within 30 seconds. I finally decided it was time to drop the pan and check things. I was disappointed to find that the rear main cap babbitt is crumbling around and near the front thrust surface. The rest of the babbitt in the engine looks great and it runs great. The question for everyone is, is it time to have the whole bottom end rebuilt or should I send the cap to Bill Barlow in Oregon to have that repoured? Thanks in advance.
Matthew
 
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One concern might be:

"If the rear crankshaft bearing cap on the bottom is crumbling, in what kind of condition is the immediately above half of the rear babbitt bearing, which was poured in the block?"


Also, how much end play do you have in the crankshaft?
 
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The front side of the thrust takes very little load. If it were mine I would just remove the broken babbitt and replace it with a brass thrust bearing, I've even seen it done with smashed copper tubing. The new thrust can be held in place with 6-32 brass flat-head screws or pins made from brazing rod. Don't worry about the upper part in the block. If this was on the rear of the cap it would be a different story.
 
As far as I can tell, the top half of the bearing looks to be intact. Of course it's difficult to see too well, but the upper thrust surface appears to be ok.


H. L. Chauvin said:
One concern might be:

"If the rear crankshaft bearing cap on the bottom is crumbling, in what kind of condition is the immediately above half of the rear babbitt bearing, which was poured in the block?"
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I considered this, but was concerned because the cracks in the babbitt extend about halfway back in the bearing surface. Do you think this would hang together if I just remove the front thrust surface? Also, when you say that the thrust can be held in place with screws, do you then drill small holes into the cap itself for the screws?

Jim Brierley said:
The front side of the thrust takes very little load. If it were mine I would just remove the broken babbitt and replace it with a brass thrust bearing, I've even seen it done with smashed copper tubing. The new thrust can be held in place with 6-32 brass flat-head screws or pins made from brazing rod. Don't worry about the upper part in the block. If this was on the rear of the cap it would be a different story.
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Jim Brierley said:
The front side of the thrust takes very little load. If it were mine I would just remove the broken babbitt and replace it with a brass thrust bearing, I've even seen it done with smashed copper tubing. The new thrust can be held in place with 6-32 brass flat-head screws or pins made from brazing rod. Don't worry about the upper part in the block. If this was on the rear of the cap it would be a different story.
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That is I would do also. I have seen a bunch with the same problem that you have. I think the babbitt that they are using now days is not the best for the thrust. If you ever have your engine redone, you might want to put in inserts and be done with it.
 
I would send it out and have the bearing repoured. I have changed out 4 rear main bearings with good used ones, that was 8 years ago no problems sinse.

mike
 
I was thinking the same as Mike, but I'd probably send another cap or pay the core and keep the cap you have now, just in case the new cap didn't work as well as you'd hoped.
 
Since this is under discussion,I bought a rear cap with babbitt and the cap itself is thinner than what I have for other engines. It has Ford script on it so it is an original. Does anyone know if it could be for a very early engine? I did get lucky and it's the right size babbitt! I had the same problem as Kippmw. Bill
 
Bill in Al said:
Since this is under discussion,I bought a rear cap with babbitt and the cap itself is thinner than what I have for other engines. It has Ford script on it so it is an original. Does anyone know if it could be for a very early engine? I did get lucky and it's the right size babbitt! I had the same problem as Kippmw. Bill
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Yes, sometime in 1929 (about April I think) the rear main cap changed from a forging to a cast iron part. Cast iron is weaker, so the mounts were made thicker.
 
You can use the early forged rear main cap on the 30-31 engines but you will need the shorter main cap bolts. I've used the modern style main bearing bolts with special washers on the last two of my engines. Berts would probably have the original bolts for the forged caps. The forged caps are said to be better and can be tinned, the cast iron style are not tinned.
 
Thank you to everyone for your insights. After thinking it over and talking to a friend who does inserts, I've decided to just convert to insert bearings and be confident in the bottom end. Thanks again!
Matthew
 
Parades and long lights with the clutch pedal depressed are very hard on rear thrust bearings.

Any long periods with clutch depressed should be avoided.

I have seen inside several inserted engines that Bert's rebuilds during the rebuilding process.

They use the full 360 degree brass insert, in the cap and block, for the rear thrust. Nice job on rebuilds including new inserted rods.
 
Purdy Swoft said:
You can use the early forged rear main cap on the 30-31 engines but you will need the shorter main cap bolts. I've used the modern style main bearing bolts with special washers on the last two of my engines. Berts would probably have the original bolts for the forged caps. The forged caps are said to be better and can be tinned, the cast iron style are not tinned.
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Purdy,I ran new threads down the bolts to get enough threads to tighten the cap.About 1/2+" was enough.Also are you going on the Talladega Track tour?Thanks Tom for that info.--Bill
 
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