Topic: 75 cb750 k5 to 78 k8 cyl parts compatibility

[reed]

I have a 78 750 k8 that threw a rod ruining the barrel and crankshaft. I purchased a 75 750 k5 engine with decent compression to replace the blown engine. On cyl 1 and 2 compression was a little low about 120 psi. Cyl # 3 and #4 were about 140 psi. I brought her home anyways and ran a leak down test on the new engine to see where it was losing compression. Long story short... shes leaking through the valves. Intake on #1 and Exhaust on #2. Can I take the good head from my k8 and put it on k5 barrels? Whats about the cams? Also my new engine is missing an alternator will the k8's fit?


Thank you.

[reed]

compression update
#1 125 on leak down test air escapes from intake valve
#2 135 on leak down test air escapes from exhaust valve
#3 145
#4 150

Does anybody have anything to say about the cyclinder head switch?? k8 head on a k5 barrel.
And is it worth it? Should I just run this engine as is?

[GammaFlat]

The head on a K7/K8 has a different shaped chamber (and smaller I think).  Check the volume of the chambers - some have talked about using a piece of plexiglass and an accurate way to measure the trans fluid you put in the chamber...

[reed]

Using the pistons from the k8 engine will elimate any clearance volume issues. Will the oil passages line up using the k5 barrels w the k8 head?

[scottly]

compression update
#1 125 on leak down test air escapes from intake valve
#2 135 on leak down test air escapes from exhaust valve
#3 145
#4 150

Does anybody have anything to say about the cyclinder head switch?? k8 head on a k5 barrel.
And is it worth it? Should I just run this engine as is?

I'd be tempted to just check valve clearances, then run it for a while before re-checking the leak-down test.
Yes, the K8 alternator will fit.

[reed]

Too late to run it. I tore down the second engine top end. Piston #1s rings were welded to the piston. With a 1/8 inch layer of soot built up on top on the piston. Must of been left on either intake or exhaust srtroke for some time..
Valve seats show some carbon build up but are not in bad condition. Exhaust valve guides show .020 of play. I went ahead and grinded the valves and ordered new exhaust guides and piston rings. Anybody got a tip on how to remove stuck piston rings?
Thank you for your response.

[willbird]

Soak it in Kroil ? Never tried it, but Kroil is some cool stuff, the oil the kreeps. I got some Gunk hydroseal II to clean some carbs, it is some amazing stuff too....although people do not seem to like it as well as the original.