Turn the machine up and forward, until the bottom plate is accessible, and prop it up so it doesn't fall down either way. Then see if there is a bottom plate on it. If so, remove it. Inside there you will see the aluminum disc, and the driven disc which has a rubber tire on it. If the tire is worn and cracked, it must be changed. A careful look-see in there will show how to replace the disc. Before you slide the disc off the cross shaft, mark the shaft so you will know where the disc goes when replacing it. If you don't mark it, you will run the risk of having too many speeds in reverse and not enough in forward gear. There will be several bolts thru the side plates, where the cross shaft is bolted in. Do not change the position of the disc on the bottom end of the crank-shaft. Thats the vertical shaft sticking down out of the bottom of the engine. (I say this because--well---most folks will know what i mean!)