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To continue the saga:
I was gifted a complete '92 CB750 Nighthawk with the top end apart. The PO said he stripped out a plug thread and instead of repairing it, he bought a used cylinder head. Well I'm putting the bike together and now it's time to put on the head. Both heads have all the valves still installed. Do I need to lap the valves if they haven't been apart?

Also, the "refurbished" head has evidence of detonation, I think I know why that bike was parted out! This is the refurb head that doesn't have the stripped plug threads, but it does have this damage on one cylinder:




I can fix the original head with a helicoil, if chasing the threads with a tap doesn't work.

Is this head in the picture decent to run?

coffee_brake wrote:Do I need to lap the valves if they haven't been apart?

Turn the head so that the combustion chamber is facing up, then pour some kerosene into it. Let it sit for a few minutes, if the kerosene does not drain out through the valves then they don't need to be lapped.

coffee_brake wrote:Is this head in the picture decent to run?

Nasty looking damage on the one side, it almost looks like something was bouncing around in there. Do the kerosene test and if it passes it should be good to use. As for the spark plug hole, you can try using a thread chaser on it but generally speaking once they're stripped, they're stripped. The threads are very fine and even if you do manage to clean it up there won't be enough strength in them to withstand the torquing down of the spark plug. The heli-coil repair is the best option.

Thanks, had never seen this kind of damage before and wasn't sure if it was a show-stopper or not....

Go with Gummi....

If the valves hold solvent, run it. I would feel better with the damage to the squish area on that head than a repaired plug thread in my motor.

OK, thanks again.

I've watched my mechanic buddy repair several stripped plug threads with Helicoils, he explained why they sometimes fail and also why his helicoil repairs *don't* fail. Years later these same bikes are still coming to his shop for routine service, so I think they must be good repairs.

He'll guide me through this repair; and I'll bring him some extra helicoil inserts. And if I make a mistake and it won't hold, I have this extra head!