rear main oil leak

[Model A Apprentice]

Recently purchased 1931 pick up, which already had extensive engine rebuilding completed, and puddles oil after being in motion. It does not leak when up on jacks! Odometer shows 620 miles but... Oil return tube is clear. Oil fill cap is good. The engine has a balanced crank, aluminum pistons, new wrist pins, and insert bearings that are 20 over. The center main has been modified for lateral tolerance control. There is 0 lateral movement. Tolerance under the rear main is "slightly" over at .006. Center and front mains have not been checked.

The slinger was NOT machined down to accept modern oil seal. The existing aluminum rear oil seal rotated on the crank and was removed. The seal shows some grinding or chewing on the forward top external arch. (Like a dog chewed it!) A replacement aluminum seal will not rotate up into the slots and around the crank.

Short of pulling the engine and sending it out, any suggestions would be greatly appreciated.

[Mike V. Florida]

.006 is way too much. Clearance should be about .002. You need to remove some shims.

[Model A Apprentice]

The truck has insert bearings. So there aren't any shims. Mystery how an engine with this much work and only 620 miles could be so out of tolerence.

[pat in Santa Cruz]

Who built it? Might not be a mystery at all. Sounds like you need to get some inserts another .010 smaller and regrind the crank. Just out of curiosity, is there a fillet radius on the main bearing journals or did the grinder grind it with square corners ?

[Mike V. Florida]

Originally Posted by Model A Apprentice

The truck has insert bearings. So there aren't any shims. Mystery how an engine with this much work and only 620 miles could be so out of tolerence.

Ok, .006 is too much clearance you need to get different inserts.

I think you are assuming it was within tolerance to begin with. I find it real difficult to believe that the rear wore .004 in only 620 miles if you had oil in it.

[bogdonj]

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[Model A Apprentice]

Sorry for my ignorance but I am new to engine's internal componants and the site. What is a fillet radius?

[Model A Apprentice]

I agree that being out .004 on this clean of an engine with such extensive work is puzzling. But I only have a few miles on the truck around the neighborhood. First thing off the transport truck we changed the oil. Old oil was clean and free of metal shavings. No idea.

Question is, can I do sometning to fix the oil leak with out sending the engine out.

Does anyone know if, when using inserts, is the aluminum rear seal still used? Does the use of inserts allow a higher quantity of oil to reach the rear seals and "overwhelm" an aluminum seal? Snyder's catalog has a bronze thrust washer to be used in place of the aluminum rear seal when using inserts. Is the bronze thrust washer used as a better seal or is that for minimizing lateral movement?

The old aluminum rear seal rotated around the crank in the seal channels without any problems. (Engine and crank are still in the truck.) But the replacement aluminum seal will not start into the seal channels. Is there a trick to getting the replacement seal started into the channels?

[pat in Santa Cruz]

The replacement aluminum seals are from Argentina, and fit very tight. An original fits better, but ordinarily you must remove the crank to do it.

There's no way you will find inserts that fit what you have now unless the crank was ground for one brand and fit with another, but from what I know, even the various brands vary by only .001. You'll probably have to get the next size under and fit the crank to it. There are 3 types of bearings commonly used: Kings, sold by H&H, Antique Engine Rebuildings, and MG bearings. MG bearings will be 2 shells per saddle sometimes 3 in the rear cap. Also used have been Wisconsin engine bearings and there are a few others I forget now. You should probably take this to a machinist who knows A engines. There is no indication where you are located or someone could recommend one. If you have any idea who rebuilt it, you should contact them even if you end up not trusting their work, just to see what you can do, if anything. With .004 too much clearance, it will leak and vibrate, eventually the bearings will fail if the crank does not break first. I would be suspicious of the rest of the work if such an oversight as a bad crank was in any of my engines. Thats why I asked previously about the fillet radius. Inexperienced crank grinders do not leave a sufficient fillet in an A crank, and they will break at the center main without it.

The bronze thrust washer is made by Antique Engine Rebuilding, and is to control thrust forward and back when the babbitt thrusts break off..


In fact, if you cannot locate the original builder or an experienced machinist near you, AER would be a good place to send your engine.

[Model A Apprentice]

There are 3 types of bearings commonly used: Kings, sold by H&H, Antique Engine Rebuildings, and MG bearings. MG bearings will be 2 shells per saddle sometimes 3 in the rear cap.

The rear main cap does have two saddles as opposed to the longer inserts pictured in both the Mac's and Snyder's catalogs. So by that I would assume they are the MG bearings you reference. What and where would I locate MG bearings.


Thats why I asked previously about the fillet radius.

Can you describe a fillet radius and I will look again in the morning. Or, If I could figure out how to post pictures, I would post several. I appreciate your time and experiance.


In fact, if you cannot locate the original builder or an experienced machinist near you, AER would be a good place to send your engine.

I am in New York. Schwamm's is nearby and was recommended by Bratton. Texas T also came highly recommended.

If I need to send it out I will, but I would like to understand the problem first.

[Model A Apprentice]

Pictues of rear Journal and Main cap

[Model A Apprentice]

rear crank and seal

[James Rogers]

Sounds to me like the line boring was done too big and the crush is not right. If this is true the only way to remedy the problem is to use shim stock behind the bearings. When the caps are torqued down without shells in them the bore should be 1.7705 to 1.771. Thrusts on the center bearing web is all wrong! Ford didn't do it that way and neither will I. I use a full thrust on the rear of the rear main and a half thrust on the front of it. I would like to see pics of the motor and diagnose the problem.

[pat in Santa Cruz]

yep, that's what an MG bearing looks like, but you need the numbers on the back to be certain. As I said, there are a lot of the lesser common bearings too. The fillet radius is at the corners of the crank journals. The journal has a small (about 1/8th) radius up the side to the sides (thrust area on the back main and rods) instead of a sharp 90 degree corner. This strengthens the journal and prevents cracks from forming at the sharp point made by a 90 degree cut.

Schwalms has an excellent reputation. good choice. James Rogers has a good reputation too

[James Rogers]

This is what it should look like. Some do it slightly differently but the same idea and materials. Notice the recess in the cap for a thrust like the block rear.

[pat in Santa Cruz]

Thrusts on the center bearing web are what I usually find coincidental with the crank ground without a fillet radius, among other bad machining. Almost always on engines from the same builder. BTW, is your crankcase painted inside with reddish paint??

[Model A Apprentice]

rear main journal and rear seal seen peeking out

[Tom Wesenberg]

Farmall Cub bearings also fit and you can double them up or even use 3 for the rear main, but I would go with the one piece bearing made for the block, such as James pictured.

Have you used a micrometer to measure the rear main diameter? Let us know what it reads, or have someone measure it and let us know.

[Model A Apprentice]

rear journal close up

[Model A Apprentice]

center main and crank