MY two cents is that you never know when it will break,you can check the back lash to see how much ware it has but that only tells you change it now,if you let it go you are sitting on a time bomb, MB. is looking out for there rep. by giving you a change time.Our 420 has 160 + and the original chain,but I dont jump on the gas for any reason,I hope mine will last a long time yet BUT I chose to take that chance.JNT