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Different upper and lower case halves?

clebay

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I've read a few places that you cannot use different upper and lower engine case halves, at least for my 80 CB750K. I understand they were machined after being joined. Is there ANY way to mate them/use them together? Has anyone ever done it successfully? My lower case has a crack in it, and the replacement I purchased is only the lower half (I know, rookie mistake...).

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They are matched machined pairs. Having said that I'm sure some idiot has tried it. Begging for spun bearings there. The bearings not only have to have the correct clearance to the spinning part but they rely on a certain amount of crush between the bearing shells to keep them in place. Lose control of that by less than a thousandth and you have a spun bearing.

An engineer could likely do it but only if the case variance runs one way, going the other way will be whopping trouble even for him. If you can't grasp bearing preload while still having clearance then you will be lost doing it. And having the preload slightly high holds case halves apart to leak oil with no resolution.

You have the problem of each half hole being too high or low in each half and forward or backward too. Out of that I'm sure by luck some mismatches would work but you don't want to spend money to blow it up instantly to find out. Early DOHC are already too expensive to rebuild as it is.

You can check the cases to see if you luck out, but you will need to be better than good to do it. Calipers are not close enough for that work.
 
Thanks for the detailed response... that’s what I was afraid of. I guess I have a new parts holder while I wait for matching cases to come on the market. Thanks for your help!
 
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