Sooner or later, it boils down to what direction you're going to go when your Mustang's rear axle wears out. Some decisions are straightforward. If you have a six-cylinder Mustang, stick with the factory 7 3/4-inch integral carrier rear end. Perform a rebuild with fresh bearings and seals, plus a ring and pinion if needed, and you're good to go for another 100,000 miles or more. If you're running a stock or mild small-block, all you need is the 8-inch removable carrier rear axle that your Mustang came with. Again, fresh bearings, seals, and cogs and you're ready to ride. Depending on wear and noise, sometimes you can get away with reusing the gears.
But what if you want to step up the power with horsepower and torque that will make metal shavings out of your 8-inch? The remedy is a tough and time-tested Ford 9-inch rear axle.
Which 9-inch differential you choose depends on how much power you intend to put through it. If you prefer to stick with a factory 9-inch rearend, you have options, including those with the "N" nodular iron case that was used in ultra high-performance Mustangs like the Boss 302. There's only one differential case we would discourage using - the waffle casting known as the "WAR" case. This is the original 9-inch differential case used underneath '57 through early '60s Fords, Mercurys, and Lincolns. It closely resembles the '67 and later 8-inch casting, which is also waffled in appearance. At a glance, the WAR case also looks like the "N" nodular iron case, also waffled in appearance with cross ribbing. According to professionals, the "WAR" case just isn't as durable as later cases.
And don't go on visual appearance alone; always check casting numbers and date codes because not all "N" nodular iron castings have an "N" in the iron. If you're planning no more than 350-400 horsepower, you can go with the C7AW 9-inch casting, which is quite durable if you're on a budget and cannot afford an "N" case.
When building a 9-inch, Bill Thomas of Bill Thomas Enterprises suggests a strict regiment of blueprinting. Bill closely inspects and dresses ring and pinion surfaces in order for both to mesh nice and square with each other. Any imperfections or debris can cause the ring gear to sit cockeyed on the differential hub. You won't see it with the naked eye, but gear teeth will wear unevenly and burn up.
When You Need Stronger
When a factory "N" or "C7AW" 9-inch case won't cut the mustard, you need both strength and a lightweight design. Strange Engineering has a variety of 9-inch castings you can work with depending on budget and power expected. There are two terrific Strange cases for classic Mustang 9-inch axle housings - the Lightweight 2014-T-6 Aluminum 9-inch casting and the Pro Iron 9-inch D4515 casting.
The Pro Iron N1905 or M1906 case can be called an "N" case on steroids because it offers unequalled strength. It is 8-percent stronger than an "N" case, according to Strange Engineering. This makes it perfect for street-driven powerhouses. It also offers outstanding strength for all-out road or drag racing. All you have to do is choose the right axles-either 31- or 35-spline-for a bulletproof package.
Mustang Rear Axle Dimensions
If you're searching for a 9-inch rear axle assembly, measure from axle flange to axle flange, plus leaf spring perch dimensions center to center, in order to choose the right axle for your classic Mustang. Remember, the Mustang's track became wider with each generation, as shown below. Also remember that even though axle flange to axle flange dimensions may be the same, leaf spring perch distances may be different depending on where the axle assembly came from.
 This is Ford's "N" nodular...  This is Ford's "N" nodular iron differential casting, which first saw use in 1963... |  ...and was used extensively...  ...and was used extensively in high-performance Ford applications. |  Here's an inside look at the...  Here's an inside look at the Ford 9-inch four-pinion differential with stock Limited-Slip package. When we speak of an 8- or 9-inch rear end, we're talking about ring gear diameter. |
 Knocked apart, the 9-inch...  Knocked apart, the 9-inch Ford is simple in scope, consisting of the main casting, pinion support, ring and pinion package, side and pinion bearings, pilot bearing, yoke, and the four-pinion differential clutch pack. |  This is the more common "C7AW"...  This is the more common "C7AW" case that first entered service in 1967, good for up to 350 horsepower. This Currie Enterprises' "C7AW" case went into Project Reclaim at Mustangs Plus. |  Here are two types of locking...  Here are two types of locking differentials side by side. On the left is a forged aftermarket unit, a thicker, more durable piece with a sprag-style clutch that hooks up when we get the power on. On the right is a stock four-pinion clutch pack unit as found in Limited-Slip and Traction-Lok differentials. The factory Traction-Lok unit can take up to 350-400 horsepower. If you need a good life insurance policy, opt for the aftermarket unit. Street drivers should not be fitted with a Detroit Locker (not shown here), which is horribly noisy in turns. Detroit Lockers are ideal for race cars, but not street cars. |
 The pinion pilot bearing is...  The pinion pilot bearing is the weakest link in Ford removable carrier rear axles. Failure occurs when either the bearing and/or casting around the bearing fails during a burst of power. This is an "N" case pilot bearing. Observe the absence of sufficient iron around the bearing. There's not enough to provide good pilot bearing support. The news gets worse for "WAR" and "C7AW" 9-inch cases, which have even less meat around this bearing. |  When building a 9-inch, Bill...  When building a 9-inch, Bill Thomas dresses all mating surfaces for a flush fit. |  Use a thread locker, such...  Use a thread locker, such as Loctite 262, to ascertain fastener security. Conventional and locking differentials have two different bolt lengths. Don't get that mixed up. |