stop a rear main leak

[George Miller]

Rear main leaks come up all the time. I will share my 50 years of experience on a model A engine. If you do all this right it will not leak more than a drop or two when you shut it down after a run.

1= rear main cap must be real flat where it mates to the block

2=so called rear main seal has to be flush with the block, can not go deeper. Put some sealer on it before you put it in.

I like to use Syniders rear main thrust A-6334 I think others sell this also.
Some of the original seals are over size from the crank rubbing when the crank was not on the right center line.

3= Take the soft plug out of the rear main cap, run a drill through the oil drain hole. Do this with the drain pipe in place. You might have to use a die grinder or file to open up the hole because there is a good chance the drain pipe is blocking the hole. This is one reason a lot of A's leak out the rear main. Do not run the drill through the other end of the cap. stop were the hole stopped originally.

4=Put a chamfer on the bolt holes in the block for the rear main bolts. then use a little sealer around the holes when you put the rear main cap on. This will act like a O ring and stop oil from leaking around the bolt and coming out around the heads of the bolt. Also put thin film of sealer on the mounting face of the cap.

5= I like to have .0015= .002 clearance for the rear main.

6= I like .004= .006 end thrust any more and the crank goes back and forth throughing the oil from the crank when it is all the way back.

7= Do not forget the rear of the cam shaft, that has to be sealed good. I always use sealer around the cam hole. Plus on some engines the bolt holes go inside the block so I always put sealer on them.

Of course your crank needs to be on the original center line. A lot that I have seen are not.

If I forgot any thing I will add it later.
I know every one will not agree but I has worked for me for many years.

[James Rogers]

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George is correct on all counts. I personally don't do the chamfer but, to each his own. All the others are good advice and spot on.