We replaced the cam chain in my '03 YZ 250F (which I owned for 3.5 years) around the 2 year mark, just for general preventative maintenance. Recommended by DRN's engine guru Rich Rohrich. For $30, it's worth it for the piece of mind, as I have heard they can stretch/wear out, just like any other chain. It was pretty easy to replace, once we figured it out. You did have to pull the flywheel (maybe stator too?) if I remember right to get to the bottom holder. In the 2 years I owned it, I think I did rings after 1 year, and a full top end and cam chain at 2 years, and another ring job around 3 years. One set of clutch plates a year too. Valves never moved from new. :cool: