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This is the 10:31 ring and pinion set for Hewland and Webster Mk8, Mk9, and Formula Mazda gearboxes. The ring gear uses the current 7/16" mounting bolts (some early Webster differentials were drilled for 3/8" bolts). Replaces Hewland part number HC8221C.
Setup Note: Webster pinions are marked with the Setting Distance (for example, "SD 2.377"). This is different than Hewland, who mark their pinions with a clearance number (the clearance between the pinion gear and the Hewland setting jig). Webster does not assume the use of the Hewland jig, so they measure from the head of the pinion gear to the centerline of the stub axle.
The Hewland setting jig measures 4.71" diameter, or 2.355" from the axle centerline. Subtract that number from the Setting Distance to find the clearance (in our example, 2.377 - 2.355 = 0.022").
You can make your own pinion setting tool if desired. Just subtract the radius (not the diameter) of your own tool from the Setting Distance to find the desired clearance.