I have a 02 400 exc that runs perfect. I ride in eastern Kentucky. Real tight woods riding. I have had my torn down once and had new rings put in it. That was about three years ago. Other than that all I have done is changed the oil, plug, and valve adjustments. When should I think about having the motor pulled down and a new piston and rings put in? I ride quite a bit. However, I do take care and maintain my bike.