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Question:

What is the proper way to measure an axle for a 12bolt posi?

Do you measure from the bottom of the stud plate or the top of the stud plate? To the top of the splines or to the tip of the axle where the c-clip goes on (which is just past the splines).

Answer:

If you are replacing the original axle with a stock part, then use the part number-that is the best way to get the correct axle. If you are purchasing from an after market company, they will want to know the year and make of the housing, as you know, there are small differences, due to the track of the various years and vehicles, and each maker has a different way to measure. For example, the 66 through 67 chevelle 12 bolt is about 1.5 inches more narrow than the later models. But If you are trying to junkyard a 12 bolt, a tried and true American hot rodding tradition, then you measure from the tip of the axle spline side to the outside of the stud plate. And remember, check for interchangeable parts with the Howard guide, not all 12 bolts will interchange.
May 28, 2018

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