your screename doesnt jive with your questions :smile: the chain tentioner is part of the bike it keeps the chain tight its between the clutch and the back wheel sprocket. You should be able to pop it on simply by setting the bottom of the chain on the sprocket and roll the bike forward just watch your fingers! but to keep it from falling off again you should make sure the tentioner is doing its job and the rear wheel is on straight and tight and make sure the engine bolts are tight too