Yamaha KT100 ignition driver hub only. Fits any 1-1/4 (1.25) Hegar gear sleeve 18, 19, 20, 21 or 22 tooth. Sleeve can be changed simply by removing the nut and washer, sliding the sleeve off and replacing with another. Attaches to the magnet of the Yamaha motor. Hegar recommends applying with blue Locktite on crank nut.