The pitch diameter of a gear is the diameter of the gear's pitch circle, which has a radius equal to the distance from the gear's center to its pitch point. The pitch point is the point wherein two gears interact with each other. Unlike in threaded fasteners, where external and internal threads should have the same thread pitch and pitch diameter to mate perfectly, gears can mate together even when they don't have the same pitch diameters as long as they have the same gear pitch. Read more