Cool Fact:
The title of third member derives from it being the third member in a twowheel- drive vehicle's drivetrain; the first being the engine and the second the transmission.
Being frugal is not just our nature, it's a way of life. So it's no surprise when we needed a rear axle for a full-width mini-truck project that we hit the junkyards in search of the legendary Ford 9-inch. A staple in the off-road world since its first appearance in the 1950s, the 9-inch has enabled rockcrawlers and racers alike to benefit from Dana 60-like strength, but avoid the weight drawbacks of heavier rear axles. Not only are they relatively easy to find, but undoubtedly one of the most heavily supported axles by the aftermarket industry.
So what makes the 9-inch the perfect axle for the backyard builder? Engineered with a removable centersection (erroneously referred to as a third member at times), you can easily swap it with an upgraded unit like the one we got from West Coast Differentials. Stuffed with 5.13 gears and a Detroit locker, the unit comes ready to drop into the factory 9-inch housing. No fiddling around with dial indicators or shimming pinions, simply remove the original diff, clean and inspect the housing, then slap your new one in. Other than being a little heavy, it's truly easy swap-if you can change your oil, you can replace a diff.
From coast to coast these beauties are available for 20 to 150 bucks in a variety of widths and perch configurations. We plucked ours from an early '80s Bronco that was missing the original center but still had the big bearing 31-spline shafts fully intact along with the factory drum brakes. Although the 9-inch is no Dana 80, we have plenty of confidence that the factory shafts and new centersection will handle the 37-inch-tall tires with all the gearing and engine we can throw at it for a long while to come.
Housing the ring-and-pinion, carrier, and bearings, the centersection is the heart of every drop-out-style axle. Since our junkyard housing didn't come with one, we gave the differential experts over at West Coast Differentials a call and had them build one of their drop-in assemblies to our specs. With plans for 37-inch tires and various terrains in the future, we went with an always dependable Detroit locker and 5.13 gears to make sure we didn't lose any power through our 31-spline factory shafts. | 
The unit is secured to the housing by 10 potruding studs. As long as the studs are not damaged you should be fine with simply cleaning them up a bit and reusing them. Once you have cleaned the studs and the mounting surface, use a high-temperature rtv to seal the assembly to the housing. | 
Our junkyard housing had been sitting gutless for some time, so naturally the bearings and seals were pretty much toast. Although this is the big-bearing 9-inch, there are a couple of bearing variations that you may have. Once you've removed the factory set, you should be able to match up the bearing at your local parts store or 4x4 garage. |
The 9-inch axles can still be found in many of the early fullsize Ford trucks and Broncos across the country. If you are lucky enough to pull one from a truck that's still running, just unbolt the four-bolt backing plate and inspect the bearings and race. You'll be looking for scorching, burn marks, or pits to indicate if the old set is too damaged to reuse. | 
Depending on what decade you get your 9-inch axle from, you will either have 28- or 31-spline axleshafts. The later fullsize trucks and Broncos will all be a bit wider than the earlier versions (closer to 62 inches), and should have the stronger 31-spline shafts. There are a few variations in the housing itself, but this will not affect the centersection. | 
While you're perusing the junkyard for that perfect 9-inch, keep your eyes peeled for an Explorer 8.8-inch rear axle. More specifically you are looking for the rear-disc backing plate and brakes, as they will bolt directly up to the big-bearing 9-inch. If you are running the 5-on-4.5 pattern, the 8.8-inch rotors will work just fine, but if you plan to run the bigger 5-on-5.5, you will need to source a custom set of rotors from the aftermarket industry. However, it is still one of the cheapest and easiest disc-brake conversions you may ever do. |