Ford Differential Builder's Guide

Whenever you're building a hot restomod, thoughts always eventually turn to the rear axle. At one time, choices were simple: 8- and 9-inch removable carrier differentials and housings, two-pinion or four-pinion, large bearing or small bearing, standard case or "N" case. But have you looked at the rear axle selection for vintage Fords lately? It has never been more plentiful. Not only do we have a choice of 8- and 9-inch rearends, we also have the option of the Fox-body Mustang's 8.8-inch integral-carrier rearend. With all of these choices out there, it's time to understand how to make the best one in our quest for reliable rear-axle performance.

Ford Rear-Axle Quick Reference
It's easy to understand Ford rear-axle identification. Five basic types have been used in Fords, Mercurys, and Lincolns since 1957. Here's how it plays out.

Fox Momentum For A Classic
Did you know you can purchase a late-model 8.8-inch integral-carrier rear axle and bolt it into your classic '65-'73 Mustang? Perhaps you are scratching your head, wondering why we would suggest the 8.8 for a vintage Mustang. Why not install the brute 9-inch beastie? The 8.8-inch Fox-body Mustang axle offers us as much strength as the 9-inch, with less internal friction and less weight. Drive Train Specialists can ship you this axle assembly, ready for bolt-in. All you have to do is pay for it, then tell them where to ship it. See Drive Train Specialists for more details.

Two Patches Of Rubber
You may upgrade your Ford rear axle with the installation of a Limited Slip clutch pack during a rebuild. This can be accomplished just as easily with a 7-1/4-inch integral-carrier axle as it can a 9-inch, four-pinion unit. Currie Enterprises, as one example, has Limited Slip differential kits for all types of Ford axles, including the 8.8-inch, Fox-body Mustang axle.

9-Inch Fact And Fiction

The 9-inch Ford differential needs little introduction for seasoned enthusiasts. Most of us understand that it is the strongest differential ever made for Fords and other vehicles alike. NASCAR racers love it for its integrity--which means it sees use with the Chevy boys as well. The 9-inch Ford outperforms the GM 12-bolt rearend by a wide margin. It has found a near equal in Chrysler's Dana rearend, which is a proven performer. But when push comes to shove, racers opt for the 9-inch Ford. Street-rodders love the 9-inch Ford for its easy-to-service design and ability to take punishment.

The rough and tough 9-inch Ford was introduced in 1957, and it didn't change much in appearance throughout its production life. What it did do was improve as horsepower ratings and demands increased. Likely the greatest changes came during the early '60s, as the horsepower wars heated up.

In the beginning, there were 28-spline axleshafts and small axle bearings. And this remained the basic standard throughout the 9-inch Ford's production life. As this design evolved, Ford made solid improvements, such as a four-pinion carrier, Limited Slip, Equa-Lok, and Traction-Lok, larger axle bearings, and 31-spline axleshafts. Ford also produced a much stronger nodular-iron case, known as the "N" case, for high-performance applications. You'll find the "N" case in Boss 302s and Boss 429s, for example. Sometimes, you'll find it in 428 Cobra Jet "Drag Pack" Mustangs as well. The earliest "N" applications were 406 and 427ci big-block Galaxies in '62-'63.

The 9-inch Ford differential is easy to understand. Choosing the right one for your mission is also easy. It just isn't always cheap. Much depends on what you find, where you find it, and what your needs are. The most common 9-inch differential case is the C1AW-C, C4AW-A, C4AW-C, and C7AW-E casting, easily identified by its single-rib design. This casting has a single rib that runs from 12 to 6 o'clock, flanked by three cross ribs. Think of this casting as the basic 9-inch carrier, used from the early '60s through the early '80s. This is a durable 9-inch case, used in a host of Fords ranging from big-block Fairlanes and Mustangs to fullsize Fords and pickup trucks. The most common version of this casting is C7AW-E, used from '66 through the early '80s. Expect to find this casting in a lot of 9-inch housings from '66 through the early '80s. Hot spots are big Fords and pickup trucks.

Another fairly common 9-inch case is the "WAR" casting, with two 12- and 6-o'clock ribs, very similar in appearance to the smaller '67-and-up 8-inch case, and the heavy-duty 9-inch "N" case. The "WAR" case doesn't employ the high nodular-iron witnessed with the C7AW-E case mentioned earlier. The "WAR" case pinion pilot support doesn't have as much iron either, making it more prone to failure. The "WAR" case is more common with the earliest 9-inch axles from '57-'65. The C7AW-E single-rib case is common from '67 and up, making it more plentiful. We have also seen the single-rib casting with different numbers--C1AW-C, C4AW-A, and C4AW-C--indicating an early '60s origin. This makes the C7AW, C1AW, and C4AW single-rib case quite common.

There is also the "WAB" case, which is similar to the "WAR" case in appearance. Where the "WAB" case differs is differential side bearing size (3.063 inches in diameter). The "WAB" case isn't shown here. The most common 9-inch differential side bearing size is 2.892 inches in most applications.

Another area of interest is the front pinion support. We have seen several variations and casting numbers in our travels. Expect to see some pinion supports with a guard cast into the piece above the yoke. This guard is designed to keep the spinning driveshaft yoke away from the chassis bounce-back bumper whenever the vehicle bottoms out in a dip. With some vehicle applications, this is not a concern--in which case, the cast-in guard will not be present. Mustangs, for example, need the pinion support with the cast-in or a bolt-on guard. The cast-in guard was not used when the bolt-on guard was used.

The most common pinion supports we see out there don't have the cast-in guard. Expect to see C0AW-A, C5AW-A, C6AW-A, C7AW-C, D2AW-A, and D2SW-C. The C7AW-C pinion support has the cast-in guard.

The highly sought-after 9-inch "N" case was born of a need for a super-durable 9-inch casting that could take the punishment of both NASCAR circle track and NHRA drag racing. The "N" stands for "nodular iron" case, indicating the solid iron material used. At a glance, the "N" is the quickest way to identify the nodular-iron 9-inch case. But, not all "N" cases received the "N" marking in the casting. Look for the C2AW-A, C4AW-B, and D0OW-B casting numbers when the "N" is missing. Another strong clue for the "N" case is 31-spline axle hubs and shafts. But, finding 31-spline hubs and shafts does not always mean you've found an "N" case. This is why your homework must be thorough. All of the elements of an "N" case must add up.

Another clue with "N" cases is the front pinion support. Like the "N" case, with its high nodular-iron content, the front pinion support was also specific. This pinion support was called the Daytona pinion support. They have the same outer pinion bearing as all 9-inch differentials. They get meaty inside, with a larger inner pinion bearing. There is also a lot more iron inside to support that larger inner bearing. Look for C5AW-C, D1OW-C, and D2OW-C casting numbers. Some have the cast-in guard. Some don't.

If you're out there searching among the ruins and happen to find a 9-inch differential sporting a 9 3/8-inch ring gear, don't be surprised. The 9 3/8-inch case looks more like the "WAR" case mentioned earlier. It has two 12/6 o'clock ribs, like the "WAR" case. It also has a different pinion support, with a C8AW-B casting number. None of these castings are interchangeable with the single-rib or "WAR" cases. And, truthfully, this is a differential to stay away from.

The 8-Inch Light-Duty

When Ford was planning the '62 Fairlane, with 221/260ci small-block V-8 power, it found an immediate need for a light-duty, removable-carrier rear axle that could stand up to the torque. The 8-inch, removable-carrier rear axle is a scaled-down version of the 9-inch axle. Ford used the 8-inch axle across a huge variety of applications in compact and intermediate-size vehicles from '62 through the early '70s. You can expect to find the 8-inch in most small-block applications (except High-Performance) and some six-cylinder packages. For example, Mavericks and Comets equipped with the 250ci six all had 8-inch rearends with five-lug hubs. The same is true for the Granada and Monarch with 250ci sixes.

Although enthusiasts have generally viewed the 8-inch rear axle as the one to toss as horsepower has increased, there is renewed interest in this Ford axle these days. Currie Enterprises, for example, is marketing an aluminum 8-inch carrier, which is lighter and much stronger than the factory cast-iron case. You can fill it with Limited Slip, also from Currie, and have a terrific differential that will stand up to greater amounts of power.

If you're building an 8-inch differential, there are a couple of issues to be mindful of in the process. First, choose a '67 and up case, which is stronger than those we find from '62-'66. The '62-'66 8-inch case has horizontal ribs across the front of the carrier, with less iron around the pinion pilot. From 1967, the case has both vertical and horizontal ribs, plus more iron around the pinion pilot, which makes it a stronger case.

All 8-inch axle assemblies were fitted with 28-spline axleshafts. Did you know the 8- and 9-inch axles used the same axleshafts? The 31-spline axle, available in the 9-inch, will not work in the 8-inch. A large-bearing 9-inch axle will not fit the 8-inch housing either.

Small Bearing, Large Bearing

How do you identify a small-bearing 9-inch housing from a large-bearing housing? You can do this without having to pull the brake drum and axleshaft. The small-bearing 9-inch housing has 3/8-inch brake backing-plate bolts, which take a 9/16-inch socket. Large-bearing, 9-inch housings have 7/16-inch brake backing-plate bolts, which need an 11/16-inch socket. Axle bearing size was determined by gross vehicle weight. Large-bearing 9-inch housings were used in Fords with a higher gross vehicle weight.

Another quick way to identify small and large-bearing housings is brake size. Most small-bearing housings had 10-inch drum brakes. Large-bearing housings had 11-inch drum brakes.

Lightweight 8-inch Carrier

Currie Enterprises introduces the new Alumin8 differential carrier for 8-inch Ford axlehousings. The Alumin8 looks sharp, weighs less, and gives you the strength of a 9-inch axle without the weight and drag. You may polish the Alumin8 or you can install it as is. The Alumin8 is made from super-strong 206 aluminum alloy, which means great strength, without the weight. This little gem is a Currie exclusive.