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Allis Chalmers Discussion Forum

AC B main/rod bearings

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10-04-2005 08:00:01

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I'm building the engine on my 1939 B and have a question on the crank bearings. The engine originally had shims on the caps, and the service manual talks about shims too, but when i look up new bearings to buy them, they list oversizes (.010, .020 etc). I can't imagine that after getting the crank ground to clean it up that I'll need to shim it too.
So what's the story? If I get .010" taken off the Rods and Mains, can't I just buy .010" oversize bearings and be done with it (like on most other engines)? Of course I would plastigauge it...but is there something different about these. I just don't want to make a dopey mistake and figured I'd ask before I ordered parts for it. The crank is getting cleaned up today.
Joe R

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10-04-2005 17:53:54

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 Re: AC B main/rod bearings in reply to JoeyR, 10-04-2005 08:00:01  
I am not going to give you all the details of how to do the job here, but here is the basic idea. If you decide to do it you can contact me and I will go into detail. Basically you need shims, as was said, to make the hole round and to fit the bearing shell. The bearing shell needs to be "crushed" a few thousands of an inch to make sure it is a tight fit in the bearing bore, so it won't move or spin. You need to check the bearing with plasti gage, and if it is too loose you need to reduce the shims and file the bearing shell to maintain the proper crush. Working down until you get the right clearance and crush. It takes a bit of time to do a nice job. You don't just bolt it together. It needs to be checked to see that it spins after each bearing is in, even though as the spin gets harder and harder as you put pistons in, (ring drag), you still need to check. Again, e mail me if you want to do the job, just put something in the subject like Allis B bearing shims, so I won't delete your e mail with all the span I get.

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10-05-2005 05:47:14

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 Re: AC B main/rod bearings.THANKS! in reply to wdtom, 10-04-2005 17:53:54  
Thanks everyone for the help. I appreciate it. I am going to get the crank ground .010", buy .010" oversize bearings and then fit them with shims. Although it looks like it will be a bit more of a project than a more modern engine, it still beats the scraping and adjusting that goes along with poured bearings.
Joe R

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Dick L

10-04-2005 09:54:47

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 Re: AC B main/rod bearings in reply to JoeyR, 10-04-2005 08:00:01  
Like Chris said, the rod and main caps do not have a round bore when tightened down without shims. The oversized bearing sizes have to do with the inside diameter and has nothing to do with outside of the bearing halves.

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Chris (OR)

10-04-2005 08:45:16

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 Re: AC B main/rod bearings in reply to JoeyR, 10-04-2005 08:00:01  
Well I drug home a '39 B several years ago. It was very seized and I knew I would have to rebuild everything....

But after I talked to folks, I didn't bother with the shims. I was getting the block remachined anyway so I had the main bearing housings cut round. While they should have been .006" -.000" short, they were egged (top to bottom) almost .006". The crush of the extra length (bearings) didn't really seem to show up when I gauged it after the crank was ground and block holes cut round.

My reasoning behind what I did was the fact that I am not going to try to run this thing 200 hours a month (like a real farmer would) and modern bearings are harder than those produced 60 years ago. You (most likely) will not put the wear and tear on the beast that the shims were designed to compensate for.

Yes, it was a bitc* to start the first few times (I still have only crank starting on it) with everything nice and tight. But I didn't have to drag start it the first time after the rebuild. I now have about 75 hours on it and it's wonderful. I can get it to catch on the second crank 95% of the time...

If you have further questions, ask. I may have already been there...

Chris Horn
Scappoose, Oregon

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