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Main Bearing Clearance

Posted: August 21st, 2017, 1:07 am
by NZAA
Hi There,
my father and I had the A engine from our truck rebuilt and after approximately 1500 miles it developed a deep knock. I have since found that the rear main bearing had too much clearance (more than .006") i am in the process of shaving down the caps to try to get a bit more life out of it and am wondering what the recommended clearance is????

Re: Main Bearing Clearance

Posted: August 21st, 2017, 8:42 am
by Bob C
.0015-.002 should do it. With only 1500 miles on the rebuild why are there no shims left??

Bob

Re: Main Bearing Clearance

Posted: August 21st, 2017, 12:16 pm
by NZAA
The person that Rebuilt the engine along with many others that I've talked to was against installing shims at the lineboring stage for reasons i'm unsure about as they would have made this process much easier and would have saved me from shaving the caps down resulting in oval holes for next time it is rebuilt. The other thing he was against and i'm sure it has also contributed to the increased clearance in such a short time was peining the babbitt. When we were assembling the engine we found that by pushing our thumbs on the sides of any of the bearings in the block or rear main cap (the untinned ones) oil could be seen squishing out between the babbitt and block/cap.

Re: Main Bearing Clearance

Posted: August 21st, 2017, 1:07 pm
by Bob C
Sounds like that engine in not long for this world.

Bob