dirt wears the pins and bushes allowing the chain pitch to increase. this wears the sprockets more at the top and hooks them over. get an old std chain and lay it out on concrete. pull it along by the end and mark the position of the ends. then push 1 end gently back toward the other end keeping it all straight. it will shorten by a cm or two as the pin wear is pushed back on it self. o rings don't do this. the rollers wear through and fly off, the pins become partially seized, but the pins never stretch. your sprockets stay in shape till the teeth wear through