Topic: 1978 cb750 Cylinder head swap question

[arncruz]

I have 1978 CB750K motorcycle that I bought recently and was able to get it started.  However, after the first start, I noticed oil leaking from the bottom.  after close inspection and removal of years of oil build up, I find a crack in the crankcase right adjacent to the main sprocket.  It was probably from loose chain hitting the crankcase.  Regardless, I was able to find a parts bike but it is a 1978 cb750F.  The crankcase looks good so far but the cylinder head has several broken fins.  My question is can I swap the CB750K cylinder head and place it on the crankcase and jugs of the cb750F engine?  Has anyone done this swap?  I tried to do a search on the forum but no results.

Thanks for any help in advance.

[hsas.69]

Should work no problem.

[Old Scrambler]

except the pistons WILL hit the valves

If you switch the pistons and closely watch the o-ring placements, the K-head can be set on the F-jugs..........but check the oil flow to the head before you run the motor. The F oil feed is different.

P.S. - If the crack in the cases is just that, it could be repaired............

[arncruz]

I finally got the donor engine from a parts bike I bought and  I took a closer look to see if there was any oil leaks which I did not see.  However I did see a small crack on the top crankcase by the clutch.  I don't see any oil marks or seepage around the crack.  I'm wondering if I need to replace this too.

[madmtnmotors]

How bad are the broken fins? I might be inclined to run the F motor as-is and repair/rebuild the K motor.

[CB750F2]

The domed shaped pistons of the F will hit the valves of the K head and also the four of the oil drain back holes of the F cylinders will need to be modified to allow the oil from the K head drain back to the crankcase. You will avoid all this if you install the head, pistons and cylinders from the K motor. However, as Old Scrambler has suggested why not repair your K crankcase. The crack on the F can be repaired as well.
A temporary repair of your K crankcase can be done using JB Weld. Pat

[koendd]

do make sure you use the K pistons and modify the F cylinders oil return as said before.

I ran my F2 for about a 1000miles when I took it apart to install new pucks due to oil leak.

turned out I had a K head on F cylinders/pistons. (nothing modified)
also F2 cam.

result was a burned cam and valves hitting the pistons (valves were not bent though...)

it actually ran great but not sure how much longer that would have been the case..

[arncruz]

madmtnmotors, I like that idea.  I will probably do that and repair the K engine.  The fins are not too bad looking as I thought.

[madmtnmotors]

I usually drop the oil pan and make sure the oil pump screen is clear as cheap insurance before pressing a used mill back into service. It can also be an indicator of the condition of the engine internals. I've cleaned some pretty nasty/gloppy screens and the engines ran well, just not so much if the pump screen is "all choked up"! 

[hsas.69]

Sorry for the miss information. Didn't realize the dome was that large on the F pistons.