I am getting ready to replace the top end on my 450 this winter during the off season. Do I need to replace the crank and bearings or just stick with the top end? Valves,piston, rings etc. It is an 05 I bought used in 07 it had appx. 20 hours on it when I got it, and now probably in the neighborhood of 45-50. It still has great compression, I am new to the 4 stroke scene, how long do the 4 strokes last before needing rebuilt? I am very meticulos on oil changes and air filter cleaning after every ride etc, check the valves at 30 hours and they where still spot on. When I tear it down I want to do it right if I am going to spend the time and money to rebuild it, I am a diesel mechanic so the expierience thing is not an issue just looking for some feedback on intervalves on rebuilds for a 4 sroke.