B Engine questions
Printed From: Unofficial Allis
Category: Allis Chalmers
Forum Name: Farm Equipment
Forum Description: everything about Allis-Chalmers farm equipment
URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=184571
Printed Date: 10 Sep 2025 at 6:53am
Software Version: Web Wiz Forums 11.10 - http://www.webwizforums.com
Topic: B Engine questions
Posted By: AcFordHawk
Subject: B Engine questions
Date Posted: 13 Nov 2021 at 11:28am
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Engine is disassembled for rebuild and curious about the rod bearings. The old bearings seem to have quite a bit of wear on the sides and not much on the other areas indicating to me incorrect set up. I don't have a close up of bearings and there is some scraps and scratching in the non- contact areas. Manual info is interesting on setting clearance with the shims. Non-running when purchased so no knowledge of any engine noise. With new bearings and may be turning the crank, are the shims still necessary to set proper clearance? Here is a pic of the rod bearings and can see the shiny wear areas on the bearing joints.  Probably have to R&R one rod - piston is installed 180 out by comparison to the other three marks and indications. 2nd piston from the top is one in question. Bottom piston is just turned 180. I assume the notch on the bottom of the piston is to face the front of the engine similar to V8s I have rebuilt.  There is enough wear on the crank for cause to turn to an undersize of .010 - micrometer is indicating at or possibly just over the limit for standard size bearings. Have to measure the taper on the bores and see if sleeves will need replaced. Don't think the manual has a limit on taper. ------------- IB (restoring), MF GC1705, MF 2705E |
Replies:
Posted By: steve(ill)
Date Posted: 13 Nov 2021 at 12:18pm
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Lots of dirt and crud in the oil over the past 70 years.. That gives scratches and shinny spots.. all the bearings are shimmed to get about .002 clearance... If you are going to turn the crank, you have to option of having it machined with NEW bearings and NO shims.. You can do that on the crank, the rods, or both... your choice and your money.. Have to get the bearing journal diameters and how much ( if any) out of round to make your decisions.. Nothing wrong with shims, it ran that way for 70 years.. Nothing wrong with eliminating them, if the crank needs turned. ( if you eliminate shims you have to rebore the block and rods / caps to size. ) Been several years since i did a B motor, but i remember the connecting rods are offset to one side so the dont hit the block.. Look in your book.. It may be #1 and #3 are set facing opposite from the bearing as compared to #2 and #4... I hone the cylinders and put in new rings.. Have had a few that were a couple thousands too much taper ( per the spec) and i reused them with success.. What your really worried about is the ring gap.. remember every .001 wear in the BORE, gives you .003 extra clearance in the ring gap. Either way, pull the sleeves and install new o-ring seals on reinstall. Remember, the motor is 125 cubic inch and puts out an AMAZING 25 HP ( max) at low RPM... so everything dont have to be exactly to "SPEC".
------------- Like them all, but love the "B"s. |
Posted By: steve(ill)
Date Posted: 13 Nov 2021 at 12:35pm
 ------------- Like them all, but love the "B"s. |
Posted By: AcFordHawk
Date Posted: 13 Nov 2021 at 1:11pm
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Steve, thanks for the input. Did see the serial number indicates a C engine installed. Think it has been worked on as the front section of the frame is cut off and indications of RTV sealant in various points. Contemplating just rings and bearings with a polish on the crank to clean up a little. Not really going to be used for much other than acreage work. Good idea on the O-rings for the sleeves - need to check the block for cracks - don't see any visually. Curious on having to rebore if going with no shims - egg shaped to accommodate the shimming or just that much difference on outside diameter of shim bearings vs non-shim bearings? Have to relook at the manual I have - may have skipped over a paragraph or two or is omitted. ------------- IB (restoring), MF GC1705, MF 2705E |
Posted By: PaulB
Date Posted: 13 Nov 2021 at 2:52pm
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Shims or no shims, either way will do just fine if done correctly. If you wish to go without Shims. #1 Get the crank reground correctly. #2 have the block line bored for correct clearance. #3 have the big end of the rods resized for proper clearance. Do the job right and then when your grandkids work on the engine in another 70 They won't be wondering how this or that was done. I've taken many of these apart that someone just did it their way and not as the Allis engineers designed it to be.
------------- If it was fun to pull in LOW gear, I could have a John Deere. Real pullers don't have speed limits. If you can't make it GO... make it SHINY |
Posted By: steve(ill)
Date Posted: 13 Nov 2021 at 3:56pm
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Contemplating just rings and bearings with a polish on the crank to clean up a little. Not really going to be used for much other than acreage work. Good idea on the O-rings for the sleeves .... That gives you a very economical overhaul.... Rings, bearings, gaskets and a valve job.. I have done several.. As long as you are CLOSE to the spec, things work out great... There is a slight possibility that the block has warped a thousandth or two.. When your doing the main bearings, rotate after you shim EACH bearing.. If it gets tight with proper clearance, you might have to add a shim to the center to get clearance on the "high side" for the crank to rotate smooth. ------------- Like them all, but love the "B"s. |
Posted By: steve(ill)
Date Posted: 13 Nov 2021 at 5:59pm
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thanks for the input. Did see the serial number indicates a C engine installed The very early B motors were BE xxxxx serial numbers and 116 cubic inches... When they came out with the C model tractor, they increased the displacement to 125 inch and BOTH the B and C engines use the SAME motor.. called a CE xxxxx. ------------- Like them all, but love the "B"s. |