Fits these chain drive Chamberlain Models: 1100, 1200, 2100, 2200, 4100, 4200, CG40, CG40D, CG42D, PD100, PD200, PD210, PD210D, PD212, PD212D, PD600, PD610, PD610D, PD612KLD, PD612K, 5100, 6200, 7200, 7220, 7320, 7420, 7520D, 4893OD, HD200D.The Large Gear is 2 3/4" x 1/2" wide, with a 1/2" bore. The worm gear is 1 1/8" OD, and the teeth are 1 1/4" wide Sizes are approximate. If you have a belt drive I recommend getting part number number cham 41A2817 and replacing only the gear.
See video for short tutorial on how to replace your gear assembly.