im in the process of stripping my old nc29 engine to get the gearbox out. on initial inspection of the engine, i new there was a problem with the top end, its number 2 cylinder. the exhaust valves are bent, and if i look down the barrel the piston isnt there. its at the bottom of the sump in a million peices and i think some went out the exhaust. i did a compression test on it a few weeks prior to it breaking. i wasnt racing the bike, just on a commute back from work. the compression was 200psi on every cylinder, and i think that is very good. i was wondering what could of caused the piston on number 2 cylinder to dissintergrate. lol.

How longs that piece of string !!!

Leaning of cylinder causing detonation
Poor lubrication to underside of piston
Exhaust valve stuck open
Foreign body in cylinder / transfer ports
Wear and tear on piston crown
Dropped valve

I'm all out for now !

With both your exhaust valves bent I would suggest the piston went first, after that your on your own.

Check everything surrounding for damage before you rebuild, smashed pistons I have only had the misfortune to come across on high boost fords and that's literally because they were not up to the job.