1979 CB750 engine architecture


New Member
A 1979 Honda CB750F recently showed up at the shop (with title, score!). It's in pieces and in pretty rough shape. The engine had water in it and the pistons had to be broken up to get them out (done by the previous owner), but the heads are in good condition (just valve work). The previous owner managed to find a '79 K motor, which came with the bike. I also have a set of CB900 cams.

My question is what are the differences in the F and K motors? I know the heads were a big one. Is there anything else? My options are to just use the K motor as is, or use the F heads on the K motor with the 900 cams, or find '79 F cylinders and build the original motor.

I have a stock 1980 CB750F and am working on a 1981 CB750K project, so I'm sort of torn on which direction to go. Any insights or suggestions would be helpful.
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Over 1,000 Posts
I didn't know the heads were different. I recently rebuilt an '82 CB750F engine, and one thing I know for sure is that CB900 cams drop right in - that's what I did. Something something lobes keeping valves open/closed for longer/shorter periods = more hp.


New Member
stick a 823 kit in it with cb900 cams makes for a fun bike
That is under consideration. CycleX is 'near' by, so I might be calling them in the spring. I did manage to find '79 F cylinders and pistons for $55 and free shipping, which is insane given it's literally a lump of metal. So I might be building the F motor up to be a big bore conversion.
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New Member
The cylinders and pistons have arrived. They are a bit manky, but so are the heads and crankcase.

Oddly this is motivation to get the Kilo done so I can dig into it.

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