Despite last being offered under a production vehicle in 1986, the Ford 9-inch continues to live a healthy life thanks to its light weight and abundant ground clearance. Not all 9-inch components are equal though when it comes to strength, or price. To illustrate the bevy of options, we recently took a factory third member and compared it to an aftermarket third member from G2 Axle & Gear and a money-is-no-object third member from Gearworks. All three were fitted with ARB Air Lockers, but even the selectable locking differentials are not created equal.
At the beer budget end of the spectrum, we used a factory 9-inch third member and the original pinion support and 1310 yoke. To that we added 4.88 gears and a 31-spline ARB Air Locker (PN RD119) to work in conjunction with factory big bearing axleshafts. The standard case is not as desirable as a nodular case, but it is far more common. If you set up the gears yourself, you can have a similar setup for around $1,200 that will live a long life behind a mild V-8 and 37-inch-tall tires.
With more engine, bigger tires, or a heavy rig, the factory parts might not be strong enough. The 9-inch benefits from huge aftermarket support though, so much so that it can be overwhelming. We built a third member with entirely new parts from G2 Axle & Gear and ARB that is significantly stronger than the factory third due to better materials and bigger bearings, but it also cost us twice as much money. We spent our money on a G2 third member (available in either aluminum or nodular steel), stronger “Daytona” style pinion support, a G2 ring-and-pinion, and a full install kit with Timken bearings. The G2 Axle & Gear components were paired with ARB’s RD99 Air Locker, which is similar to the RD119 except it accepts larger, 1 1/2-inch-diameter, 35-spline axleshafts, making it well matched to chromoly axleshafts and fabricated 9-inch housings that are regularly found under buggies and hardcore rockcrawling rigs.
From left to right are the Gearkworks third member, G2 third member, and a factory Ford third member. Note how the Gearworks and G2 thirds have additional webbing to prevent any flexing. They are also built from nodular iron, which is considerably stronger that the typical factory cast third member.
If you own a Trophy Truck or Ultra4 car, or simply must have the very best, it’s time to look at Gearworks components. The Gearworks third member is designed to accommodate a larger, 10-inch ring gear for increased strength with no penalty in ground clearance. The case uses huge, 4-inch bearings than can accept axleshafts up to 2 1/2 inches in diameter.
We bolted the 10-inch ring gear to ARB’s RD99CE competition Air Locker. “We love a challenge,” ARB USA President Doug Pettis says. “There’s only so much room inside of a 9-inch housing, so in order to increase the case strength by 290 percent we had to get creative with regards to design and materials.”
The result is the strongest selectable locker ever offered for the 9-inch axle thanks to deeper tooth engagement and forged internals. Again, the price roughly doubles from the G2 components and RD99 Locker, and is four times higher than our rebuilt junkyard third member.
Which third member is right for you depends on a variety of factors, including your budget and intended use. The beauty of the 9-inch is that you can mix and match parts under your rig, upgrading as components fail or your budget increases. Even though the 9-inch hasn’t been found under a new Ford in decades, this axle isn’t going away any time soon.
The third pinion bearing is one of the reasons the 9-inch is so strong, but the factory case (right) has little material around the pocket bearing. The G2 third member (center) and Gearworks (left) housings have far more material around the pocket bearing. The limiting factor becomes differential clearance.
Different sized bearing caps are available from the aftermarket. The stock case uses 3.062-inch bearings that are well matched to the original 28-spline or 31-spline axle shafts. The G2 third uses 3.25-inch bearings that accept 1.5-inch, 35 spline axle shafts, and the Gearworks third uses huge 3.812-inch bearings that support up to 2-inch, 45 spline axle shafts.
G2 Axle & Gear are best known for their JK Wrangler ring and pinion sets and Placer Gold axle shafts, but they also make a whole host of Ford 9-inch parts, including nodular and aluminum third members. The aluminum third member is one third lighter than the nodular third, but trades weight savings for impact resistance.
G2 offers full install kits with high quality Timken brand bearings for a variety of axles, including the 9-inch. While ring gear bolts are included, we had fitment issues with our ARB Air Locker that required twelve-point fasteners to clear the differential case.
Here is a closer look at that third pinion bearing that the 9-inch is famous for. In addition to having more material, the type of material is also a factor in strength. Gray iron has a tensile strength of 30,000 psi, while nodular iron is 65,000 psi.
Here you can see the downside of additional material around the pocket bearing. Interference between our ARB Air Locker and the housing meant that we had to carefully remove material from the third member for proper fitment. We took a little material at a time with a grinder and checked the fitment often.
The “Daytona” pinion support from G2 (right) has more ribs for increased strength, and uses a 50% larger pinion gearing for greater durability. This is an inexpensive bolt-on upgrade that we recommend for any 9-inch third member.
While these two ARB Air Lockers look similar, there is a huge difference in strength between the two. How big? The RD99CE (Competition) locker on the left is 290% stronger than the standard RD99 thanks to forged internals, improved lubrication, and deeper clutch tooth engagement.
One of the unique features of the RD99CE is locking tabs that thread into the carrier to keep the ring gear bolts from backing out. In the past you had to add safety wire to accomplish this, but the locking tabs are much quicker and reusable.
The G2 ring and pinion (right) is forged from 8260 steel and is a good choice for most rockcrawling rigs. The Gearworks ring and pinion (left) is made from AMS 6265 steel that offer increased strength and shock resistance. The Gearworks ring gear is physically larger, and it is heat treated for off-road racing, rather than the “soft” gears that are common in drag racing. The tradeoff is the price tag, as the Gearworks gears cost three times as much as the G2 gears.
All 9-inch gears have a deep hypoid cut due to the way the pinion gear mates with the ring gear at the bottom of the ring gear, but not all pinions are created equal. The G2 9-inch uses a 1.31-inch, 28 spline pinion (right), the same as stock, and the Gearworks pinion (left) has a huge 1.75-inch, 35-spline pinion shaft.
The Gearworks pinion support (left) picks up where the Daytona pinion support leaves off, constructed from 2024 T-351 aluminum and designed to maximize oiling for off-road racing applications. It accommodates larger bearings and bigger mounting hardware as well, but again the tradeoff is cost.
ARB’s RD99CE competition locker is the choice for Ultra4 racers such as Shannon Campbell. In races such as King of the Hammers, the selectable nature of ARB’s Air Locker is a huge advantage, but reliability under harsh conditions is the most important factor.