The manufacturer states the larger Diameter (same tooth count ???) is intended to allow for a worn motor case, specifically the upper and lower holes that locate the gear. The blue plastic plug (removable) they say aids in stabilizing the ger, I say that, along with larger diameter create more drag causing the worm gear to often climb up the teeth on the replacement gear. I'm not sold. These gears can't cost remotely close to $55.00 to manufacture, profit is not a bad word but please.....................