I have a bit of an off topic question but I'm posting here because this site if quite active. My buddy has rebuilt his KTM and now the exhaust valves are tapping the piston. The valves are original and they have not been lapped. After the rebuild the engine runs but clacks heavily. The valves were properly adjusted and the timing chain is not stretched and the cam timing is fine. My buddy wonders if it is possible that he simply overtightened the head bolts, which are the same bolts for the cylinder....he wonders if he has squashed the head gasket enough to allow for this new condition of valves hitting piston! I don't think this is possible but all other things seem correct. Is there that little space between the valve closed a the the piston at TDC? The bike wasn't doing this before. Also compression is now only 90 psi. Just wondering as I'm quite buffaloed now!