 Test radius is not the same as the operating pitch radii of two tightly meshing gears unless both are perfect and to basic or standard tooth thickness.The throat diameter is the diameter of the addendum circle at the central plane of a worm gear or of a double-enveloping worm gear.Throat form radius is the radius of the throat of an enveloping worm gear or of a double-enveloping worm, in an axial plane.