TRX400FA Clanking valve train after top end
 

Just rebuilt the top end on a 2007 TRX400FA because of excessive oil use caused by the motor being swamped and then driven after being swamped. I had to have the cylinder bored .025 over because it was beyond spec. Put an oversize piston and rings, adjusted the valves in it and buttoned it back together. Started right up and sounded good. After about 2 minutes of warming up it started to make an awful clack clack clack clack in the head but still ran fine. I shut it down thinking that I had misadjusted a valve. Checked the valves but they were at spec at .006. Started it back up again, ran fine and quite, about 2 minutes in it started to make the CLACK CLACK CLACK again, sounds like a really loose intake or exhaust rocker. So my thought is a loose valve seat, I pulled the head off and the top of the piston on the exhaust side has a small nick in the top of it. What do you guys think? Head warms up and valve seat on exhaust side comes loose causing the racket and holding the valve open slightly causing the top of the piston to nick it?

Update: Pulled the valves out of the head and the exhaust valve is indeed bent and the valve guide is broken. So my next question is, what caused the valve to kiss the top of the piston and bend it?