Gearing is the cornerstone to performance on-road and off. The numerically higher the axle ratio, the better the response to throttle input and the quicker the acceleration. Most wheelers know that and choose the axle ratios in their 4x4s accordingly when they purchase a new rig, opting for ratios like 3.73s and 4.10s over “taller” gears like 3.08s or 3.31s, which are meant for highway cruising.
Many choose 3.55s, the middle ground in gearing. All is well in that respect until it comes time to add a lift kit and taller tires. Increasing the diameter of the tires by 3 inches has the same effect as changing the axle ratio say from 3.73s to 3.42s or, in the case of our Bronco, 3.55s to 3.31s.
Stepping up to tires that are several inches taller than stock doesn’t take nearly as much of a performance toll in today’s newer fullsize 4x4s as it does on those built 20 or so years ago. That’s because the V-8s in today’s trucks and SUVs generate nearly twice the horsepower and torque as the same size engines offered back in the ’80s and ’90s. On top of that, 4x4s back in that era used four-speed automatics, while today, we have five, six, and eight speeds. Gearing is everything.
Ruben Villalobos pulled the Bronco’s rotors, spindles, and axleshafts on each side before removing these two 5/8-inch third-member support bolts on the back of the driver side of the TTB Dana 44.
Regaining Lost Performance
That brings us to rebuilding the front differential in a ’91 Ford Bronco. Its 5.0L engine and A4OD transmission really felt the power drain when a set of 33s were slipped underneath. The factory 3.55 gears, which were fine when the Bronco was running stock tires, just didn’t cut it for our intended use with the taller, meatier Pro Comp treads.
Recently, we swapped out the gears in its 8.8-inch rear differential for 4.09s and upgraded the open diff with an Eaton Posi. Then it was the time to face Ford’s Twin-Traction Beam (TTB) front differential that is found under ’80-’96 Broncos. It looked intimidating, but we soon found tackling the job wasn’t nearly as bad as we thought it would be, thanks to a few tricks Mobile Diesel Service’s Ruben Villalobos showed us as he went through the rebuild process. Yes, Mobile Diesel Service does more than just diesel tech.
The driver-side beam also serves as the differential cover for Ford’s Dana 44. When the 10 bolts are removed, all that’s keeping the differential housing attached is the silicone sealant between it and the TTB.
There’s one nuance to be keenly aware of when it comes to changing gear ratios in this era Ford truck, be it Bronco or F-series, as we learned from the techs at G2 Axle & Gear. The high-pinion Dana 44 uses two different ring gear carriers depending on the axle ratio. This is referred to as a 3.73/3.92 “carrier break.” This is important to remember because some DIYers have overlooked the carrier differences and installed the new gears, only to find the ring gear barely touches the pinion.
Carrier breaks are common. What happens is as the axle ratio increases numerically, the ring gear typically remains the same diameter while the pinion gear diameter gets smaller. At some point in changing gear ratios, keeping the two in contact means the ring gear has to be made thicker—or the ring gear mounting flange, called the deck, must be moved closer to the smaller pinion gear.
To change from 3.55s to 4.09s, like we wanted to do, required using the 3.92-and-up carrier, which placed the ring gear 0.320-inch closer to the centerline to make up for the lower axle ratio’s smaller pinion diameter. G2 Axle & Gear had both the carrier and the master rebuild kit to make the gear swap easy.
There’s no drain plug on the TTB Dana 44, so when the third member was pried loose from the RTV sealant on the back of the beam, gear oil poured out. The oil looked clean with no metal bits or discoloration from water intrusion. We added a drain plug during the rebuild.
TTB Diff Removal
Removing the differential from the Ford TTB is actually pretty simple when you think of the components. The driver-side beam is also the cover for the third member. To remove it, you have to slide out the axles. The driver-side axle is held in place by the spindle, so it easily slides out of the Dana 44 once the spindle is removed. However, the passenger-side axle is two-piece, with the outer half held in place by the spindle like the driver side. The inner slip-shaft is held in place by a C-clip on the inside of the differential, and it can only be removed once the third member is on the work bench.
Knowing this made the removal, regearing, and rebuild of the TTB Dana 44 a lot easier than we first imagined. Thanks to the expertise of both G2’s tech support staff and our mechanic friend Villalobos at Mobile Diesel Service, our big Bronco is going to feel a lot stronger when we hit the trails and backcountry roads this winter with the right axle ratio helping put the power down.
Villalobos shouldered the load of the differential like the pro he is, having spent a lot of time manhandling transmissions over the years. The passenger-side axle remained in the housing, which required the Dana 44 be twisted slightly as it was being removed.
A screwdriver rounded down and bent into a slight curve gave Villalobos just the right leverage he needed to pop loose the C-clip that holds the halfshaft in the case. The gears in our differential showed very little signs of wear.
Once the retaining clip was removed, the axle slid right out of the case. Villalobos used this time to inspect the axle bearing. It was fine, so we didn’t have to worry about waiting for a new one (the G2 kit doesn’t come with an axle case bearing) from the local parts house.
Our case was a little on the grimy side, so after the gearset was pulled, it was slid into an automated parts washer with all the other pieces for a good cleaning.
Ford used two different Dana 44 carriers in the trucks up until 1997. The new carrier on the left is for 3.92 and up ratios, while the one on the right from our ’91’s 3.55s is for 3.73 and down ratios. A tape told the difference: the case for 3.73 and numerically lower ratios measured 2.090-inch to the top of the deck, while the deck height for the 3.92 and up ratios, which we used, measured 2.410-inches.
G2 Axle & Gear had all the parts readily available for this rebuild, including a master rebuild kit, the 4.09 gearset, and the appropriate case for the lower gears. This made Villalobos’ rebuild go smoothly.
Unlike some other differentials, the cross shaft and locking pin had to be installed before the ring gear. Ruben used a small screwdriver to make sure the hole in the shaft aligned with the pin before tapping both into place in the new carrier housing. An old-school trick for getting the ring gear to slide onto the deck without using force is to heat the gear up. Villalobos used the heat of the parts washer to bring the ring gear’s temperature up.
Using a new carrier meant we had to pay close attention to the shims going back on the pinion and carrier. Villalobos used a micrometer to make adjustments in the shim packs. The G2 rebuild kit comes with a variety of shims, which saved time on the setup.
It didn’t take Villalobos long to install the new gears in the housing. He followed G2’s recommendations for pinion preload (14-19 in-lb), ring gear bolts (55 ft-lb), and bearing cap bolt (60 ft-lb) torque specs.
Gear backlash and tooth pattern is very important on any new gear swap. Villalobos set the backlash on our gears to 0.009-inch, which is within the G2 specs of 0.006-0.010-inch for this application. The tooth pattern was also on the money.
New gears! Our Dana 44 TTB’s 4.09s, new bearings and seals, all sourced through G2 Axle & Gear, will give our ’91 Ford Bronco a lot more kick on- and off-road. The quality rebuild will also mean one less drivetrain component to worry about on our 25-year-old 4x4.
One cool trick we picked up from Villalobos is making two differential housing “alignment dowels” by cutting the heads off a pair of 3/8x2-inch NC bolts and slotting the top for a flathead screwdriver. The alignment dowels are screwed into the upper bolt holes of the housing to help it slide into place without smearing the RTV sealant, which could result in leaking gear oil. It’s an old transmission specialist’s trick! Cleaning both the face of the differential housing and the TTB are critical to a good seal. Villalobos used a soft pad sander on both before laying down a thick bead of RTV on the beam/diff cover.
Another cool trick Villalobos showed us for the DIYer doing this in their garage is placing a piece of angled 1/4-inch steel plate on the bottom of the TTB. This both supports and aligns the differential housing as it’s pushed into place. Note the drain plug we added, visible above the steel plate.
Tapping For Oil
Sometimes it’s handy to be able to drain the oil from the front differential. With the Ford TTB Dana 44, to do so requires unbolting the third member from the driver-side beam that serves as the diff’s cover. This is not handy at all, so we drilled a 37/64-inch hole through the bottom of the housing, tapped it for a 3/8-inch pipe plug, and installed a hex-head plug so we could easily drain the oil. We didn’t tap all the way through so the plug would stop just before it was flush with the housing, and it couldn’t work its way into the diff itself. Now we can drain the oil by simply using a standard 3/8-inch hex wrench.
A 3/8-inch hex-head pipe plug, a 37/64-inch drill bit and a 3/8-inch pipe tap are all that’s needed to put a drain in the bottom of a Ford Dana 44 found in F-series and Broncos.
Installing a pipe plug in the bottom of your Ford’s Dana 44 during the rebuild/regearing process is an easy way to solve the no-drain issue.