47 a/c bearing shims

mpcorcoran

New User
I am rebuilding a 1947 Allis Chalmers Model B with a 125 CID engine, the question is working with or around the bearing shims used on the crankshaft bearing caps and the rod cap. any obstacle, procedures or concerns working in this area.
 
Welcome MP!

Thought someone would have posted with some hands on experience, which I don't have...

From the little exposure to shimmed bearings I have, they were limited to babbitt type bearings.

The old engines were made with poured babbitt for bearings instead of inserted bearings we have today. Replacing, or repouring babbitt bearings was an involved process, so the bearings were poured and machined with shims under the bearing caps. That way when the bearing wore, shims were removed to tighten up the bearing.

If this is what your engine has, and none of the bearings have actually failed, as in gotten so loose they pounded the babbitt away, or are worn so far that they can't be tightened up, possibly you can go back together with a simple adjustment by removing shims.

It is essential to keep the shims matched to the bearing they are on. If it has not been disassembled, now would be the time to measure the clearance. Start by removing one pair of shims (be sure the shims are all the same thickness, if not remove them in matched pairs), retorque the cap. If the bearing still has some clearance, remove another pair of shims. Keep doing this until the bearing gets tight on the shaft, then add back one set of shims. You'll want to check for clearance in several rotational positions in case the crank journal is out of round. It's a tedious process, but you don't want to get anything too tight. Loose is better than tight, and the old engines were forgiving. Once you get the clearances set, then you can disassemble the engine, keep everything matched up, do further inspection, and decide what else needs to be done.

Now, it is possible that engine has inserts, they may have been factory, or they may have been added in a previous rebuild. (If any of the babbitt bearings are beyond use, remachining the rods or mains to accept inserts is the easier way to repair it.) The problem is, if you replace the inserts, even if you match them to the crank (in case it has been ground undersize), you don't know if any shims were removed before you went into it. What you can do is reassemble the rods and mains with their associated shims, and measure the bore. If you can find the specs, go by that measurement, or check them for round. If they are not round, chances are shims have been removed. You can either try to reshim the rods, or have them reconditioned. The mains, try to reshim them, or have it line bored (expensive). In either case, if the bearings are replaced, assume nothing! Always check the clearance!

I don't know if there are any shop manuals available for that tractor, if so it would be a very valuable investment. Somewhere there are engine specs. You will need to know this and be able to measure to get the proper clearances. Plastigage is an alternative if you don't have access to micrometers.
 
Here is a link to my one picture folder with several albums of pictures of your engine. I took these pictures over the years to explain how to do certain things. The picture albums you would want to look at first would be the stuck engine and the filing bearings. I didn't give details with the pictures as I gave the detail when posting them. This is not a how to site just storage of my pictures. If you have a question I can use a picture to explain the details.
The stuck engine album was done when showing a good running engine could be had with mostly used parts. That engine has been my lawn mowing and field manure mulching tractor for seven years now.
Look Here For Pictures
 
Those engines have replaceable two-piece bearing inserts, and the brass shims are used on the bearing caps to maintain about .003 clearance to the crankshaft, checking with Plasti-gage. Shims should be divided equally (or within one) between each of the rod bolts.
 

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