just curious as to when you guys are rebuilding your 450's. I have an 06 YZ450 and i have only had to shim once at 35 hours. i currently have 40 hours on it. i change the oil ever ride and filter ever other ride. at how many hours do you guys usually replace the piston and ring? how long before the head needs to be rebuilt? im just looking at what to expect for upcoming maintenece.