Piston-Head interference Issue

[bmw633]

Hi,

I disassembled the low compression engine and took the head, camshaft, cylinder and head to a machine shop to have them do a valve job, rehone/rebore the cylinder and fit new rings in it. The machinist noticed a place on the piston where it had contacted the head. It is in the area between the exhaust valves in the flat area. He asked if I had run it without the head gasket, which I hadn't. He said there may be some vertical play in the connecting rod, and I checked and only found a little lateral play, which is within normal limits. Not knowing the history of the engines, I don't know if the heads had been shaved or if the head is warped, causing the interference.

I went ahead and gave the machinist the go ahead to do the work. I have two bottom ends. The original engine is the one the exhaust valve broke off and grenaded the engine. The other is an EBay dud that PinWorm Cycle Parts would not give me any credit on. The engine ran good for 100 miles before it started lagging, topping out at 52mph. Only 110 psi compression.

Any thoughts/suggestions? I really hate throwing money at this bike. I want it to be dependable, like all of the other Jap bikes I have owned.

Seth Anderson
Moon, VA

2001 Suzuki GZ250
2012 Honda NC700X
1991 Honda ST1100

[jonathan180iq]

A warped head could have caused the low compression, and maybe the intereference..... But I would suspect the weakness and the leak would show up near the outer edges and not something that caused contact right in the middle....

Without knowing the history of the Ebay motor, and since I don't know what the specs and tolerances are for the engine of how to check it at this point, there is really no way of knowing what caused the contact between the piston and head. If you said the lateral play was within spec, then there is no reason to question it. (Maybe check it again, but it sounds like you know what you are doing.)

The condition of the original bottom end must still be good, right? You could mate the original bottom with the new, machined top and see if you have the same issues... If there was too much lateral play in the new connecting rod, then perhaps that's not an issue with the old motor. And since you know you'll have a planed and mated surface on the new head... Well, it's worth a shot. Otherwise you're just back at square one, kind of guessing at what caused the issue to begin with. These things are usually pretty bullet proof, except for a few common issues that aren't what you are running into.