I bought a 2001 YZ426F new back in '01. Since then, I have raced it 25 or more times. I have yet to do anything to the motor. No piston, rings, valve checks, upper cam sprocket, or upper cam sprocket chain. Nothing. Changed the plug when it fouls (which is about once a month). I change the oil once every two weeks. I clean the air filter after every ride ( I keep 3 air filters on hand ready to go). I have put new knobbies on it twice. New chain and sprockets 4 times. Handlebar grips have been replaced twice. Changed my antifreeze twice. My oil filters are changed with every other oil change. I know it could go anytime. But so far, I am EXTREMELY impressed with the durability and reliability and toughness of this bike. It is by far the best bike I have ever owned, and I have owned all brands. Whattya think guys? I know it's time for maintenance. But, you know how I like to ride.