Ford 9-Inch Rear Axle - Drivetrain Tech: Number Nine
If You're Adding Big Power To Your Vintage Mustang, It May Be Time To Switch To A 9-Inch Rearend
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.