I've read alot here about valve replacement but don't see much about the actual failures. I'm working on a '05 RMZ450 but this applies to any model. Why are valves replaced? What fails? Is it typically burning at the seat? Does the stem fail at the head and drop the valve? Is the stem worn out of spec? I read about intake valve failures. I would expect to see more exhaust failures than intake as they are exposed to hot exhaust gas as oposed to cool intake air. Anyone have some insite to share?