The last piston went in at 35 hours. I bought the bike with 40 hours on it and the right intake valve zero'd(got a killer deal!). shimmed the right, but the left intake, and both exhaust valves were good. now at 62 hours, having trouble starting, left intake valve now at zero. right intake(no movement after initial adjustment and 20 hours of riding) and exhaust valves good. I pulled the plug, while not oil fouled, found oil on the threads. Also had oil on the threads when I pulled the plug just before my last ride. is it time for a rebuild or can I just shim the left intake and keep riding it? obviously if money weren't an issue, I would be doing the top end! 50/50 trail and mx riding. though I don't really scream it off the rev limiter.