What Makes The Ford 9-Inch Rear Axle So Strong?


What Makes The Ford 9-Inch Rear Axle So Strong?


Today’s high horsepower cars stress driveline components like never before. And no matter whether you’re driving a 400-hp car or a 3000-hp drag car, all that force has to get channeled through the rear axle assembly.

Over the years, many different rear ends have been tried in the quest to come up with a combination that can handle the rigors of high-performance driving. But one, in particular, has emerged as the hands-down favorite for street and race applications — the Ford 9-inch.

This proven rear-end was first introduced in 1957, and it was used in almost every kind of vehicle Ford built in the decades that followed. Although the 9-inch evolved somewhat over the years, its basic design soldiered on largely unchanged until FoMoCo phased it out in 1986.

GearFX Strong 9 safety wiring

Ford never gave this family of rear-end assemblies a formal name, but racers and rodders for ages have been calling it the 9-inch, after the size of its ring gear. And along the way, the Ford 9-inch became the go-to rear end for just about any kind of car you could imagine, mainly because of its strength — few rear-end designs can absorb punishment like the 9-inch.

So what makes the 9-inch so strong? To answer that question, we spoke to Lucas Hardin of GearFX. He gladly gave us his expert insight on 9-Inch rear ends and what can be done to fortify them for even greater capability.

Bigger Is Better

GearFX Strong 9 ring and pinion mesh testing

When it comes to performance rear ends, larger parts generally mean greater strength. The Ford 9-inch’s ring gear is pretty big as production-based rear-end assemblies go these days, and it’s right in the middle as far as classic performance rear-ends — slightly larger than that of the GM 12-bolt, but not quite as big as that of the Dana 60.

That said, the Ford 9-inch punches well above its size and weight in terms of strength, with several unique design features that make it more robust than other comparable rear-ends.

Offset Pinion

GearFX Strong 9 offset pinion

The most important design feature that adds strength to the Ford 9-inch is its offset pinion gear. When viewed from the side, you can see that the input on the 9-inch sits rather low in the housing. This may not seem significant, but it has vast implications for the strength of the rear end. “It allows more pinion-to-ring-gear tooth contact,” says Hardin. “Think of it like a fine-thread versus a coarse-thread bolt. The fine-thread bolt is going to be stronger because you have more thread engagement.”

Third Pinion Bearing

GearFX Strong 9 third bearing

In addition to the offset pinion, the Ford 9-inch also has an additional pinion bearing, which dramatically improves the overall strength of the assembly. “Where most rear ends have just an inner and an outer bearing, the 9-Inch has a pinion bearing on the other side of the teeth as well, which allows it to hold up under heavy loads because it doesn’t deflect nearly as much.”

Vast Aftermarket Development

Ford Fest 2021 1970 Ford Torino Cobra

Much of the reason the 9-inch has become the most popular rear-end for most applications is the tremendous amount of aftermarket support for it. As car builders have embraced the inherent benefits of the 9-inch design, countless manufacturers have continually developed even stronger parts for it, giving it a huge advantage over other performance rear-ends. “The 9-inch is the small-block Chevy of the rear-end world,” says Hardin. “There are a ton of aftermarket upgrades for it. You can get all kinds of heavier-duty setups versus an original.”

Different Cases for Different Cars

Although the basic 9-inch design remained largely the same throughout its long production run, there were differences introduced over the years. The most significant changes were in the basic case design. Although most aftermarket Ford 9-inch rear-ends sold nowadays don’t use any original Ford production components, their cases still adhere to the fundamental design parameters established by Blue Oval engineers ages ago. These variations in case design dictate certain limitations and advantages.

“In Ford 9-inch speak, there are different letters that indicate what case you have. An original case that came out of, say, a ‘69 Mustang or something like that would be a ‘B’ case. Those have a small carrier bearing, so they can accept only a certain size axle shaft. And again, size matters when it comes to rear-end components — smaller bearings mean less load carrying capacity and more chance for the bearing to come apart.

“After that, the ‘E’ case is kind of the middle of the road in terms of what it allows,” continues Hardin. “It’s the most popular of the 9-inch case styles, and it’s what most aftermarket cases are now. It has a 3.062” bearing, and it can reasonably accept up to a 31-spline axle shaft. From there, you move up to an ‘H’ case. Those have a 3.250” bearing. That allows you to run 35-spline axles, which can handle a lot more power.”

Adding Even More Strength

GearFX Strong 9 shimming

Although the basic 9-inch design is impressively strong to begin with, GearFX takes it further by adding several critical improvements to the 9-inch rear ends they offer. This adds considerable strength that exceeds similar rear ends offered by other companies.

“A lot of companies just use the ring-gear bolts, pinion-support bolts, and seals that come with the axle master-install kit. There’s nothing wrong with these parts, but they can be improved on — they’re generally not the best quality. Instead, we use ARP hardware, Timken bearings, and Ford Racing SVO pinion seals wherever possible.”

In addition to using quality components, proper setup can make a big difference in a rear-end assembly’s ultimate strength and longevity. Here again, GearFX goes the extra mile to offer a top-quality product that delivers maximum performance. “We take extra care and time to make sure everything’s right,” says Hardin. “Our larger competitors boast that they crank out 20 or 30 rear-ends a day. If all of our guys are working on rear-ends, we’ll probably get four or five, maybe six done per day.”

The attention to detail that GearFX lavishes on its products continues after assembly. Foremost among the extra quality-control steps they offer is their rear-end dyno service — an industry exclusive that allows them to carefully test finished gear assemblies in a controlled, measured environment. “As an extra service, we can run rear ends on our dyno to make sure that everything works correctly, everything’s happy, and everything’s going to stay proper.”

A 9-Inch For Every Need

GearFX offers a wide range of Ford 9-Inch rear ends, with strength ratings that range from 650-hp to 1000-hp and beyond. Their most popular setups are their S- and N-Series rear ends.

The S-Series is GearFX’s entry-level Ford 9-inch, but it’s far from basic. Rated for a maximum of 650-hp, it features a Strange nodular-iron case, Eaton TrueTrac differential, 31-spline axles, Daytona pinion support, Ford SVO seals, a 1350 U-bolt driveshaft yoke or pinion yoke, and ARP hardware. GearFX’s N-Series Ford 9-inch is similar to the S-Series, but with an even stronger nodular-iron case, giving it a maximum power rating of 850-hp with 31-spline axles or 1000-hp with 35-spline.

In addition to these popular choices, GearFX offers many other combinations, including aluminum cases, provisions for external coolers, custom setups, complete NASCAR-ready assemblies, and more.

But while the company provides nearly limitless choices in Ford 9-inch rear-ends, it’s not hard to pick the right one for your particular application — just call the company. They’ll be happy to help you sort out your needs and point you to the ideal setup for your car. “We want to make sure that the customer is going to be 100% happy, because that gives everyone the best chance of success.”

GearFX Strong 9 center being built


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