Only stuff that is worn badly or beyond limits (if known) the obvious stuff. I ran rollers with flat spots in the original DT and they worked fine. The flat spots indicate less weight probably too small to make a difference. I am probably not the one to ask because I have not worn anything out yet! Well, did have to replace a belt because the air inlet was blocked by mouse-hoarding birdseed causing the CVT to overheat. New Kymco belt fixed her up!