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i have a 2016 crf250r i bought new, it has 57 hours on it currently. the book says top end at 20 hours under race conditions. i race but its my first year and i am very slow, never rev limit, roll jumps so on. I check the valve clearence every 10 hours or so, only one side is dropping low but still in spec. the service manual says compression should be at 87 psi, its currently at 91. I have everything i need to put in new piston and rings, my question is should i go by compression when deciding to put in a new top end or by hours riding?