1973: FoMoCo increased the rear frame rail width behind the cab from 33 1/2" to 37 1/2" on all...except the following: 1973/77 F250 4WD's (Highboys) and 1973/79 F350's on the 140" wheelbase.

1957/67 F100 9's use the same 28 spline axleshafts / 1968/72: The are two different 9" rear axles! One type has 28 spline axleshafts, the other has a 9 3/8" ring gear and 31 spline axleshafts.

9 vs 9/3/8: Only FIVE parts are the same: Housing gasket, pinion seal, one pinion bearing and cup, pilot bearing retainer. EVERYTHING else is different!
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1973/79 F100, 1975/79 F150: Could be 28 spline, could be 31 spline. There are two ways to find out.

Pull the axle, or if the AXLE tag is still present (many are missing today), have someone with the parts catalog decode it. But, there's more to it than this.

Once the code is matched to the axleshaft part number in the rear axle parts lists, this axleshaft part number then has to be looked up in the separate axleshaft ID charts to find the spline count.

How many people have the parts catalog? How many people that do want to go thru this PITA rigamarol? I know of one, there may be others.

1948/52 F1's, 1953/67 F100's have 11" x 1 3/4" rear brakes / 1968/79 F100's, some 1980/83 F100's, 1975/96 F150's have 11" x 2 1/4' rear brakes.

CAVEAT EMPTOR ~ Let the buyer beware: 1959/67, some 1968 F100's. If Limited Slip was ordered, Ford installed Dana rear axles / After the 1st Arab Oil Embargo ended, FoMoCo installed Dana 44 rear axles in some 1976/79 F100/150's.

Dana's have an inspection cover, the 9 inch does not.