9-inch is a 9-inch is a 9-inch, right? Well, not really. I think it's safe to say most of us would answer the opposite given obvious things like the addition of a posi or a locker-type gear. Yeah, there's the difference in housing shapes throughout the years and the number of splines on the axles, which come in either 28 or 31 splines from Ford, that have transferred power to many a rear skin all these years, but besides the three-spline difference, what else is there, and why is it surprising to find out that '57-64 rearends aren't what we've been led to believe?
It all started out as we tried to install a Master Power rear disc brake kit on a '60 Ford F-100, with its presumably stock 9-inch rearend. The first thing we noticed that would send us down the road of education was that the distance from the face of the axle where the brake drum mounts to the face of the axle flange on the housing was too narrow to get the new caliper set up into...hmm. Well, who else to call for 9-inch help? Currie Enterprises in Anaheim, California, the 9-inch gurus, of course!
In the first of what would be many conversations with Ray Currie and Brian Shepard, we found out we were up a creek as far as installing the disc brake kit on the '60 was concerned, because the pre-'64 9-inches are 1/4 inch narrower than the later versions. Now what? Ray answered that question real quick. These early white-iron third members are only good up to 265 horsepower in a full-size car or truck, and many are unknowingly cracked around the rear pinion support bearing under normal duty thanks to the inferior iron used to cast them, which gives way under pinion deflection. The F-100 that's home to the rearend in question will be the recipient of a 392 Ford stroker from Smeding that will easily surpass the stock early rearend's threshold and will still need those Master Power discs out back to slow the train down. It was obvious even at this point that a new rearend was needed.
Keep reading for a quick pictorial lesson from Currie on what makes a 9-inch a 9-inch and how we got the disc brakes from Master Power hung on one.
On the back side of the flange we found "WAR" cast in raised letters, which along with possible "WAT" and "WAB" castings are all part of the weaker white-iron family. White-iron is inferior to grey-iron as far as strength goes. | 
Here is where the casting is prone to cracking. The bearing in this picture is the back pinion support bearing, which is the same size as the bearing that supports the front of the pinion in these early third members. The smaller bearing doesn't require as much material around it, and that's why the casting cracks under heavy loads. These also have wimpy spool main caps. | 
Starting in '66 and continuing to '70, this double-rib case with the big "N" was cast from a stronger nodular grey-iron and was available in big-block cars like the Shelby Cobra, Torino, and Fairlane. It's good up to the 500hp range with its bigger back bearing and has been highly hunted over the years. |
This is the Currie-built third member that's going in the '60 F-100. It has the grey-iron single-rib case used from '64-86 and is rated in the 450hp range, which is good for most any street-driven truck. It too has the larger back bearing and beefier main caps--don't believe the guy at the swap meet who's had too much coffee and is trying to tell you that you need his double-rib case over his buddy's single-rib! | 
The early cases can crack on the side of the casting under severe pinion deflection due to the inferior parts letting go. Some of the nodular cases had taller ribs on the sides of the case, but don't go on that alone. | 
This early housing was used from the birth of the 9-inch in '57 and carried through till '77 on some applications. It has always been a favorite for hot rodders due to its nice round pumpkin...come on, guys! But it's weaker than the later housings because it used a butt weld to join the axle tubes to the center. In a light hot rod, much of the early stock stuff is OK because the car would smoke the tires long before it'd break the rearend, but not so for full-size cars and trucks. |
The later housings that came out around '68 have this much of the axle tube sticking inside of the center section, which understandably proved to be much stronger, not to mention the triangulation of the center section could distribute and handle a greater load. | 
To prove the results of these findings and monitor them on their new and rebuilt third members, Currie tests everything on their rearend dyno. | 
Currie finds the parts needed to build a better rearend for any classic truck, which are soon put into action by their skilled employees. |