18 Tooth 5/8 Bore C-Sprocket. For 40,41,420 Chain. 3/16 key needed. This will give you more top end speed over the stock 10 tooth. Take off speed will be greatly reduced. This sprocket it for top speed. This will have very limited take off speed and will be hard on the clutch and belt. For some 30 series Plates a 5/8 spacer or slight modification might be necessary for chain clearance.