quick question.... on a 9" ford rear end (1972) if you dont have a step down in the outer axle tube size does that mean that you have a 31 spline rear, i know my 28 spline rear has a step down in tube size approx 6" in from the backing plates. thanks

 

65 & 66 Mustangs, Comets, Mavericks, and I believe Falcons had the step down tube. Prior to 1970 (I think) all 9" rear ends were 28 spline and from 70-72s both spline counts were used (there aging I am not exactly sure if the dates are correct or not but I am close give or take a year) so you are correct that all step down tubes were 28 spline, however Ford quit doing the step-down tube in 67 and there were lots of 9" rear ends (even the ones put in trucks) that had 28 spline until 72 so with you being on the change over year the only for sure way to know is to pull the axle and count the splines.