Top end rebuilds

[Power Stroker]

I was wondering how often I shoud rebuild the top end in my 450r, I race the local circuits here at home and plus a little free riding. Also how difficult is to do the job, I have done many 2 strokes (bikes and sleds) but I know nothing when it comes to 4 strokes to I have to mess with the cam and valves, or can i just take all that off with the cylinder?

[honduh440]

Nope have to take the chain off the cam and pull it all out in peices make sure you have a manual when doing it