well i did my rebuild this winter. did everything buy the book. Took it out ran it about and hour to break it in. did some kinda hard riding for another hour or two. I took it out the other day and was riding pretty hard. and i heard something in the motor. so i pulled in the clutch and pushed the bike back home took the valve cover off and i see my intake cam has scratches on the lobs and then look harder and all my intake valve caps are all broke. my valves on the cam side are all messed up. so i took the head off and there are marks on the piston. It is a 2006 yz250f. i put in a 08 piston and reshimed the valves and put them back in speck what did i do wrong or what went wrong. i just dont get how the timing could have been off if i ran it for 4 hour with no problems. and it went up the the rev limiter a couple times with no problems.