Redesigned ARB Air Locker for Ford 9 inch axles
A look at ARB's new Air Locker for 9" Fords Short Cuts
by: Matt Kirkwood


The Ford 9" is a challenging axle for which to build differentials. The large pinion gear and unique third pinion bearing reduce space for differential components. That's what gives 9" differentials their unique tapered look when compared to other axle designs.  ARB recently introduced a redesigned Air Locker for Ford 9" differentials which has elegantly responded to the challenges of this axle.

The Air Locker is unique among aftermarket differentials because it has the ability to operate either as an open differential, or as a fully locked spool. The locking mechanism works by using an annular piston to lock one of the side gears to the differential housing. By locking the side gear to the housing the differential loses its ability to differentiate and both axles rotate at the same speed, regardless of the traction on either side. The ability to operate fully open eliminates the handling quirks inherent to automatic locking differentials.

The new design uses a redesigned air inlet housing which seals better at highway speeds and is less likely to be damaged during installation. They also replaced the copper air inlet tube with a more forgiving plastic tube and quick connect air fitting. This design should make installation easier and reduce the likelihood of damaging the fragile o-rings used to seal the air inlet ring to the differential.

The redesigned Air Locker is available for all Ford 9" axles, but is only available with 31 spline side gears at the time of this writing. This means that Early Bronco owners must upgrade to custom 31 spline axle shafts.

We installed a redesigned model RD55 Air Locker in a '76 Ford Bronco rear axle.


Begin by removing the axle shafts and differential center section. After removing the axle retainer nuts, the axle shafts should come out with a little tugging. Stubborn axles may require a slide hammer for extrication.

After removing the center section nuts, leave two or three loosely threaded on their studs to keep the carrier from falling on your head. After placing a drain pan under the differential carefully pry the center section from the housing. The center section is pretty heavy, so it is a good idea to have help or to support the center section on a jack.

After stripping and cleaning the carrier, carefully mark, drill, and tap a hole for the air line bulkhead fitting. It's best to perform this step before installing the differential and before any bearings are in the carrier. Be sure to remove any metal particles after this operation so you don't damage the Air Locker.

After removing the axle retainer nuts, the axle should come out with a little tugging.
Hint: differential oil should not look like chocolate. Note to self: Take care to prevent contamination in the future.
Be sure to center punch the spot you wish to drill so it ends up in the right place.
The stock Ford carrier has a nice spot next to the bearing cap for the bulkhead fitting

A press makes bearing installation easy. Be sure the bearings are fully seated and don't interfere with the air inlet collar.

Press new bearings on the differential. New bearings are good insurance for your new Air Locker, and it is very convenient to replace the bearings while you have the differential apart anyway. If you don't have a press, this is a good job to farm out to a shop even if you are doing the job yourself.

The Air Locker at home in the differential carrier.

Once the bearings are in place, place the locker in the carrier housing and install the bearing races and caps. The redesigned air locker uses a special bracket on the passenger side bearing cap to hold the air inlet stationary while the differential rotates. Leaving the bearing caps a little loose now makes adjusting backlash easier later.

This pattern shows a little too much pinion depth which should be corrected by a thicker shim if the gears are new.

Check the gear setup by painting gear marking compound on at least 3 of the ring gear teeth. If re-using gears, it is best to try to match the pattern the gears had originally. With new gears, set them up to manufacturers specifications, which will mean the gear contact patch is centered on the teeth. These factory gears had a little too much pinion depth, but it was left that way since the gears have been together for 165,000 miles and changing it now might result in noisy gears.

The internal air tube is protected by a spring.

When the pattern is right, torque the bearing caps and install the adjuster retainers. Apply pipe sealant to the bulkhead fitting and screw it into the hole you tapped earlier. Cut a section of that famous blue air tube long enough to run from the differential to the bulkhead fitting. Install the tube in the bulkhead fitting using the supplied hardware, and cut the hose so that it reaches the differential but doesn't interfere with any other objects.

Careful application of RTV will prevent leaks.

Carefully apply RTV silicone to the axle housing and reinstall the differential carrier. The copper washers you probably destroyed while removing the carrier can be left out if you make sure to place a bead of silicone around all the housing studs.

31 spline axles are significantly larger and stronger than 28 splines.

As mentioned above, the Air Locker is only available for 31 spline axle shafts at this time. While this will add to the expense of the installation in a vehicle that came with 28 splines, 31 spline axles are a significant strength upgrade. This will help cut down on the number of broken axle shafts from the new found traction. You will most likely have to buy custom made 31 spline axles if your vehicle was not equipped with them, since stock 31 spline axle shafts usually have tapers that make it impossible to cut and respline them. Further complicating the matter is the variety of bearing and brake combinations that would have to be matched from the donor vehicle.

A very complete wiring harness makes wiring a breeze.

Now it's time to install the compressor and electrical system. The compressor comes with a very nice wiring harness and all the hardware necessary to mount it. The wiring harness makes installation very easy and professional looking.

The compressor fits nicely on the inner fender well of early Broncos.

The compressor fit nicely on the driver's side inner fender on this Bronco. The fact that ARB supplies all necessary hardware makes mounting the compressor one of the quickest parts of the installation.

Careful routing of the air line will prevent trail damage and allow full axle travel.

Once the compressor is in place, route the air line. The line needs to be placed out of harms way, and must also allow for axle droop. The air line is arguably the most vulnerable part of the locker, so take care in routing it. I chose to leave the extra hose coiled above the gas tank so I'd have extra hose in case it is damaged. Since this Bronco has a body lift the air line was run on top of the frame rail. This routing should protect the line from trail gotchas.

Some fabrication was required to mount the rectangular switches in an empty space in the dashboard. Note how space was cleverly left for an additional switch.

Once the air line is routed it's time to finish the electrical connections. The wiring harness comes in two pieces to allow the harness to be run though the firewall. A neat feature of the harness is that it includes provisions for two Air Lockers, making adding a front locker as easy as plugging in a switch and a solenoid. To mount the switches in the Bronco I made a steel plate to fill the empty radio spot in my dashboard. The ARB supplied switches are very attractive and include lighting for easy location at night, but they really don't fit the decor of an early Bronco.

Final analysis

The ARB Air Locker has a reputation for being mechanically strong. Based on the experience of this installation, it is clear that in order to take advantage of the mechanical strength, the air system must be carefully installed and protected. If not installed properly, the air line may be vulnerable to trail damage or wear. Also, the o-rings inside the differential can be easily damaged during installation or by debris in the differential oil. That's the bad news. The good news is that the installation was not much harder than any other differential installation and there were no problems related to the Air Locker.

The extra care required with an Air Locker is well worth it if you can't live with the handling quirks of an automatic locker or if you are using it in a front axle. There is also the coolness factor to consider. It's just plain cool to be able to lock and unlock the differential with the flip of a switch. As long as care is taken to protect the air line, and the differential oil is kept clean the ARB Air Locker will provide reliable service even in the most extreme conditions.

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