How To Choose The Right Ford 9-Inch Case For Your Project


How To Choose The Right Ford 9-Inch Case For Your Project


Of all of the many advantages of the basic Ford 9-inch rear axle design, one of its biggest pluses is its removable case. Unlike the GM 12-bolt, the Ford 9-inch has a case that can be removed as a separate unit. This allows easier servicing, because the entire differential and ring-and-pinion assembly can be removed relatively quickly — you can pick it up and plop it onto a small bench to work on it.

And besides the greater convenience that this configuration offers, it also allows many choices in materials and design features. Simply by selecting the right type of center section, you can easily tailor your rear-end setup for exactly the type of driving you’re doing.

GearFX Case Assembly

To learn more about the different Ford 9-inch cases available, we talked to Lucas Hardin of GearFX Driveline. As one of the leading suppliers of custom-built performance rear-ends, the company has a wealth of experience on the subject, and Hardin gave us the lowdown on what exactly you need for your Ford 9-inch.

Like most performance parts, strength is a key issue in center-section design for Ford 9-inch rear ends. But the importance of this isn’t necessarily for the reason you might think.

“In most situations, the weak point isn’t the case,” says Hardin. “There are typically weaker points elsewhere, whether it’s the axle shaft, the differential itself, or the ring and pinion. The case actually coming apart and breaking isn’t very common.”

GearFX 31 vs 35-spline axles

The more typical shortcoming of a weak center section is the potential for flex and movement. This can upset the carefully calibrated relationship between the ring gear, pinion gear, and other components, which can cause them to fail — often catastrophically.

Along with the issue of strength is the question of heat resistance. Different applications generate vastly different amounts of heat in the driveline. For instance, a drag car only has to endure high power for short bursts at a time, so the driveline generates relatively little heat. On the other extreme, a high-speed endurance racer like a NASCAR Cup car spends hours under high power loads and fast speeds, making heat a crucial factor.

Types of Cases

GearFX 9-in axle case

Over the years, the basic 9-inch design evolved quite a bit. This brought about dimensional changes that have important implications for performance applications. Most of the Ford 9-inch rear axles in use today are built entirely from aftermarket parts, but they still adhere to the essential design specifications set forth by Ford engineers decades ago.

These specs fall into several basic families, with letter names to differentiate them. “Most of what we do is with B, E, and H cases,” says Hardin. “Those letters denote what size carrier bearing is used. B cases use the smallest bearing. That’s what you would normally find in an older factory car that uses a 2.89-inch diameter carrier bearing. And that’s capable of handling 28-spline axles. If you bump up to the E case, it’s a much more common 3.062-inch bearing that allows you to run 31-spline axles. Most aftermarket cases now are the E case, but from there, you can go up to the H case, which uses a 3.250-inch bearing that allows you to run 35-spline axles.”

Materials Used

Ford 9-inch cases are available in a variety of materials, which offer their own advantages in terms of strength, rigidity, and heat resistance.

Nodular iron is the most common case material. Compared to gray iron, nodular iron is stronger and less brittle because of its unique spherical graphite composition. Because of this, nodular iron has largely become the standard material for most Ford 9-inch cases, in applications ranging from mild street cars to hardcore endurance racers.

GearFX N-Case

From there, the next step up in case materials is aluminum, which offers an obvious weight advantage. Although aluminum isn’t as strong or rigid as iron, it’s used effectively for rear-end cases by employing unique design elements that compensate for the inherent limitations of the material.

“Aluminum flexes and moves around,” says Hardin. “But that’s been mitigated by companies making what’s called a through-bolt case. In a typical nodular-iron case, the metal is strong enough to where you can just drill and tap for bolts to go into the case. But with aluminum, it has to be a through-bolt case. That type of case uses billet caps, along with studs that go all the way through, with a nut and a washer on the other side of the case. And pretty much all of our through-bolt cases have 3.250-inch bearings. The much greater thickness of this setup, plus the larger bearings, make these rear ends very strong.”

Quality Components

GearFX Quality Components

One of the biggest challenges in choosing a high-performance case for a Ford 9-inch is knowing whether its components are high quality - it’s almost impossible to tell a good rear-end part from a bad one just by looking at it. And this is where you can get into trouble when buying a case assembly. Poor quality driveline components can cause premature failure in any number of ways. For example, cases that aren’t machined accurately make it impossible to do a correct setup, and weak materials can cause parts-eating flex, along with many other issues.

For this reason, it’s vital that you work with a trusted supplier. To eliminate such problems, GearFX rigorously tests all major component brands and works with only the best — they refer to this process as “aggressive trial and error.”

“We’ve pretty much tested everything you can get in the industry,” says Hardin. “Even cheap stuff that we don’t use. We’ve tried it all, whether or not we’ve actively gone out and bought a bunch of them just to test, or customers have brought it to us. It’s an experience thing.”

Choosing The Right 9-Inch Case

With so many choices of Ford 9-inch case designs, materials, and suppliers, it can seem daunting to pick the right one. Fortunately, GearFX has made it as simple as possible, by offering a select range of options to suit many different types of vehicles and applications.

GearFX’s reputation for reliable, quiet, bulletproof rear ends starts with getting the right parts. These guys don’t mess around — they use only the stronger series of 9-inch case types, complemented by the highest quality components available. “Everything we do is an E case or larger bearing,” says Hardin.

With this approach, GearFX’s cases can reliably handle copious amounts of power day-in and day-out. Even the company’s most budget-friendly rear axle at the time this article was written is capable of handling 650 horsepower, with plenty of extra margin for harsh treatment. “Even if you’re putting drag radials on the back of the car with a stick and you’re racing on a prepped track, it will handle that kind of power.”

GearFX S-case

GearFX’s most popular 9-inch cases are their S-Series and N-Series nodular-iron units. The S-Series is rated at a comfortable 650 horsepower. The N-Series is rated at 900 horsepower, again with plenty of margin for extreme situations.

Besides the additional power it can handle, the GearFX N-Series case also offers a wider array of different configurations, to suit unique applications. “The N-Series is our version of the Ford SVO nodular-iron case,” says Hardin. “Typically, if you’re doing anything besides strictly drag racing, we recommend the N-Series. It’s available in 31-spline or 35-spline axles, with a Locker or Truetrac differential. We can even machine these cases to accept a pump for running an external oil cooler. That’s what we use for NASCAR setups — it’s a true NASCAR rear end for the street.”

The unique advantages of the N-Series make it an excellent choice for road racing, track-day cars, and serious pro-touring machines. “Guys that are doing all this hardcore road course stuff, they want something that they know is proven, that they know is going to work,” says Hardin. “Our N-Series ticks all the boxes for them.”

Even so, there are other choices for even more demanding users. The next step up in the GearFX line is the HS-Series. Rated for 800 horsepower, this unit uses a Ford Racing investment-cast steel case, making it approximately five pounds lighter than comparable nodular-iron cases. “We recommend the HS-Series for road racers or heavy-duty autocross guys that are trying to hit a weight class,” says Hardin. “The HS-Series gets you close to nodular-iron strength without the weight penalty. So it’s a good middle-of-the-road option for guys who want something lighter than an N-Series or any nodular-iron case, but they don’t want to go to an aluminum case because of the heat that type of racing generates.”

GearFX HS-series

For even more extreme weight saving, GearFX offers aluminum cases. This setup provides maximum weight reduction, while maintaining considerable strength, because of its through-bolt construction. The result is a case that’s excellent for racers who are trying to shed every possible pound off their car. “A lot of drag racers get aluminum cases,” said Hardin. “Weight is a big concern for them, but they also want something that’ll hold up.”

That said, aluminum cases can’t handle sustained heat very well. This makes them a poor choice for cars that endure high sustained loads. To illustrate the point, Hardin tells of an unfortunate incident that put a damper on one car owner’s fun. “We had a customer with an aluminum case that was going through Montana sustaining about 90 miles an hour, and the rear end gutted itself. The whole case got so hot that the pattern started to change, and it ate the ring and pinion alive. An iron case isn’t going to do that. It can handle the heat much better.”

No matter what type of car you have, or what kind of driving you plan to do with it, GearFX has a case to suit your needs. With their wide array of options, the biggest key to getting the right Ford 9-inch case is to simply be honest about what you plan to do with the car. From there, GearFX pros are happy to help you choose the ideal setup. All you have to do is call them.

GearFX assembly 2


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