Here's a Vintage NOS Mercury Go-Kart Clutch #GYN12-12RC35.The drum rides very smoothly on the clutch hub, held on by a snap ring. The small hole is 5/16, the crank shaft size appears to be 3/4 inch. 12 tooth sprocket, 35 chain, 1 3/8" wide drum. This unit is very heavy, hence the price for shipping. Ask questions. You will get one new in the original box,