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Connecting rod bearing IDing

Leandro

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Currently rebuilding my cb750k1, and I've gotten stuck trying to figure out what size bearings were used previously. The only thing I have to go off of is a stamped serial number on the back of the bearings. It's AP20 DOI-A STD. Any one have any idea on how to ID these just off the serial? Thanks
 
You need to check bearings by using the stampings on the rods as well as the numbers on the crankshaft. You do the same with the mains, letters on the front of the case and the letters on the crankshaft. There is a chart in the repair manuals that show how to select the bearings you need. The bearings are color coded and there are 4 sizes of bearings available for the rods and the mains. There really is no standard bearing as a motor could have come from the factory with any of the 4 colors of bearings depending on what was needed for clearance. If you dont have a manual for the CB750 I highly suggest you get one because of the information like the bearing selection.
 
I don't know of a way to id bearings from the stampings on the bearings themselves. The original paint color on the side will tell you the size, but that might be worn off.. There are markings on the crank and rods that tell you want bearings originally were used, but they could have been changed at one time or another. If the bearings look to be in good shape, I would plastigauge the bearings you have now to see if they even need to be replaced. You could use a micrometer to measure the crank and rods to figure out what bearings you need, then buy them and plastigauge the new bearings to make sure everything is within spec. You need to measure everything very accurately though (1/10,0000") so you might want to bring it to a machine shop. When you're plastigauging the rod bearings, be sure the crank does rotate at all or it'll throw your reading off.

Here are the codes for the bearing shells. Can you see any color on the old ones?

13315-425-003 A - black
13316-425-003 B - brown
13317-425-003 C - green
13318-425-003 D - yellow

I've attached the pages of the service manual that cover the bearings. I think the chart on last page has an error. Under the heading Crank Pin, I think the numbers should be 3, 4, 5 left to right. Can anyone confirm?
 

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  • cb750_service_maual_crank_bearings_1969-78.pdf
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Well dirtdigger beat me to the punch with the response, but at least I think we're on the same page.
 
And yes Travis the numbers should be 3 4 5. My manual has the same typo lol. Yes I agree that if the bearings look good and are in spec after plastigaging then there is no need to replace them. I always start with the bearing that is determined by the charts, then plastigage them and change to a different color if needed. I cant scan pictures so I will leave that to Travis:thumbsup:
 
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