Hey, I found this article on Wikipedia when i did a search for it: http://wiki.answers.com/Q/How_often_change_top_end_on_KTM_250SX "It all depends on your riding.racers change 25hrs or so.I ride every other week end 3-4 hrs,its giong on 3yrs since last top end.Buy compression tester 170psi needs top end,my compression 220.KTMs piston are forged like wisco,But austria quality.Usually bike has lost power,runs but not as peppy ,depending on compression" So, at the start its 220+PSI, and needs a rebuild at less than 170? How low should I go? 150 or what? Its currently at 192 with no carbon in the exhaust pipe, so i presume there's none in the motor either. Mike.