[roy747]

My 78 750 Super Sport engine is now on my workbench with the head and cylinders off. I have a damaged camshaft, cradle, rocker arms and valve guides. I acquired an earlier model K head which has been refreshed with new guides, seals and valve re-seating. Do I need to replace the cylinders as well when I do the head swap? I can't see any difference between the K and F2 cylinders. Is there something I am missing? Sometimes something obvious goes right by me and I need it pointed out. I also have a set of cylinders from a K model which I planned on using, but if I can keep the original cylinder, I would prefer that. Do the pistons need to be replaced as well with the K model, or will the f2 pistons work without hitting anything?

Thanks, Roy

 

[Geeto67]

Fer starters the oil pasages may not line up. IIRC the K has top end passages that the F doesn't. Don't know if they are interchangible or not, I seem to remember it not being a problem but can't remember if that is an F head on a K block or a K head on an F block. Check over at SOHC4.us

Your big problem is that yes there is a difference between pistons. You can't use an F head with K pistons because the pistons hit the valves, and using a K head with an F piston will cause the compression to drop through the floor. It will run but you loose a buttload of power. The general rule is you must match the pistons to the head.

A 77-78 F head is pretty special. Those were big valve heads with more aggressive cam. the extra power that the 77/78 cb750s make comes largely from that head (the accelerator pump carbs don't hurt either). If you are set on a K head swap, get yourself an 836 kit to make up for the ground you are loosing with those valves. The 77/78 bottom ends are the second beefiest cb750 there is (only the autos are stronger) and a 77/78 F bottom end with an 836 kit and ported K head is a pretty potent combo.