Topic: Will this work?? - CB750 F1 engine cases on a CB750 K5 Frame

[AtTheRaces]

All,

Here is the story.

I split the cases on my K5 and just like on many of these bikes there was a hole in the case near the sprocket. Th PO attempted patch this up with some kind of weld job that didn't hold up. There is also some kind of epoxy resin on the weld too??

This makes me think the PO didn't bother to split the cases and do it right. Pictures attached.

On to my questions... I found a case from a CB750 F1. Will this case work with my CB750 K5 frame? And will the K5 crankshaft work with the F1 cases, assuming i replace the bearings as needed?

IF anyone has done this i would like to know.

Thanks,

J

[shinyribs]

Yep.It'll work.   All engines from 69-76 will swap out and use the same cush drive/sprocket carrier.  The 77-78 engines will swap as well,but requires the 77-78 cush drive/sprocket carrier for proper chain alignment.  Good luck!

[CB750F2]

Yes you can but there is a couple of things that you should be aware of. 1 Make sure that you change both upper and lower cases as a pair. 2 There is a set of five capital letters stamped underneath the front of the upper crankcase - just behind the front wheel. It makes things easier if these letters are the same as the letters on the replacement cases because they are used to determine the crankshaft bearing shell thickness. If the replacement cases are different you may have to change bearing thickness. Having said all that I would check the clearances with plastigauge before doing the swap and again after and go from there. While your at it check the connecting rod clearances as well. This is all explained very well in the service manual which I believe can be downloaded from this site. Pat

[AtTheRaces]

Yes the top and bottom will be replaced together. But the stamped marks on the bottom side of the bottom case will be different.

My current marks are : BBBBB
The new marks are : AAAAA

My crank shaft is: AAAAA

This means i will be going from green to yellow bearings. The yellow bearings are smaller. I believe this is the total thickness (at 1.490 mm, thickness is the only thing that makes sense to me)??

In any case i will will check the clearances on the old case and the new case and compare. I think this will give me a good base line to start from when selecting bearings.

One other thing, I am pretty sure i will have to use the final drive, counter shaft and main shaft from the K5. There is no way these can be interchanged from the F1 to K5 due to the changes that were made for the F1 in 1976.