“My mama always said life is like a box of chocolates. You never know what you’re gonna get.”
Forrest Gump’s philosophical words were on our mind the moment Mobile Diesel Service tech Matt Johnson removed the last bolt holding the dirt-and-rust-coated cover on our 26-year-old Ford Bronco’s rear 8.8-inch differential. When the gear lube first poured out, we were elated to see it was clean and free from any obvious signs of heavy gear wear or broken bits. Then one of those chocolates that taste bad popped up as Johnson set aside the cover. The ring gear looked OK, but the spider and side gears were discolored and had a bad case of “crow’s foot.” The damaged gears would have broken under the first good shock load on the trail. Disappointing, yet not totally unexpected on a Bronco differential with more than 200,000 miles on it.
Matt Johnson lifts our rebuilt Ford ABS-model 8.8-inch differential back into the axlehousing of our ’91 Ford Bronco.
Gearing Down For Power
Although a rear diff rebuild wasn’t in the original game plan for this rig’s mild trail tune-up, it will work out for the best in the long run. The Bronco is fit with 33-inch tires, and with the stock 3.55:1 gears and 5.0L V-8 there’s not much power down low. The stock open differential makes traction in slippery situations an added concern.
The good news is that the aftermarket has the parts needed to rectify our Bronco’s woes. We contacted G2 Gear & Axle and the company sent us a 4.10:1 ring-and-pinion, Ford 8.8 Master Installation Kit, and 31-spline axleshafts. We addressed traction issues by giving Eaton a call to procure a Posi limted-slip differential.
The 4.10s will offset the taller tires, giving the Bronco the equivalent of 3.73:1 gearing. That’s a good ratio for everyday driving and light-to-medium off-road use. The G2 axleshafts will be a vast strength improvement over the OE ’shafts. The Eaton Posi will provide good traction for all-around use on- and off-road without breaking the bank.
You never know what lies inside the differential of an older truck until you get past the cover. The lube in our ’91 Bronco looked good as mechanic Matt Johnson popped it open, but the differential’s true condition was evident the moment we saw the side and pinion gears. They wouldn’t have lasted long on the trail. The first good shock load may have sent teeth flying.
Addressing Small Details
Those rebuilding Ford pickups and Broncos that have the early ABS system (mid-’80s to ’91) will also need to be diligent in checking over the reluctor gear tone ring on the differential carrier and the vehicle speed sensor (VSS) mounted in the top of the pumpkin. These older Ford trucks had drum brakes on the rear, discs up front. The ABS, along with the operation of the E4OD automatic and speedometer/odometer, are tied into the signals received from the VSS that’s reading the rotating speed of the tone ring’s teeth passing beneath it. So when it’s time to rebuild the double-eight, it may be prudent to install a new sensor and tone ring so the ABS and automatic work harmoniously.
Another little detail is to have new bearings and seals on hand to replace the old ones. The seals and bearings in older Ford axlehousings tend to wear a groove in the axleshaft, creating both a potential weak point and a leak point. If you aren’t replacing the axleshafts, an easy fix is to replace the normal traditional seals and bearings with axleshaft repair bearings. The repair bearing combines both bearing and seal into one part and moves the position of the bearing and seal outward so they ride on an unworn section of the old axleshaft.
Parts in hand, we turned to Johnson’s expertise on putting our Bronco’s ABS-equipped 8.8-inch axle back into trail-ready, daily driver shape. Read on for some valuable tips we learned along the way.
A magnet is the easiest way to remove the C-clip from the 8.8’s axleshafts. It’s what keeps the axle/wheel assembly from exiting the truck. At some point we’ll put in a kit that eliminates these keepers.
Once the C-clips are removed, the axles can be removed. We replaced our truck’s axleshafts with 31-spline G2 forged-and-heat-treated replacements, which are stronger and more durable than the OE versions.
The OE axle bearing and seal tend to wear into the factory axleshaft, creating strength and oil leak issues. If the worn axles are going to be reused, “repair” bearing assemblies are available that move the seal and bearing contact area outward so they are riding on “new” surfaces.
Cleanliness is paramount when doing differential rebuilds. Johnson cleaned the case with pressurized cleaning solvent to flush out any metal and other debris that might be hiding in the nooks and crannies.
We replaced our Bronco’s old differential parts using a G2 Master Installation Kit (PN 35-2013), G2 4.10:1 gearset (PN 2-2013-410), and a 31-spline Eaton Posi (PN 19588-010). Our four-wheeling is going to be light- to medium-level, so we didn’t feel the need for a full diff locker at this time.
One tip Johnson says first-time DIYers should follow is using a designated race punch to drive the new bearing races into the housing. These punches are made from softer metal than the races, so there’s no fear of damaging the race.
We opted to fit our Bronco’s rear differential with an Eaton Posi, which combines carbon friction discs with precision-forged gears to give great traction with smooth, quiet performance on the road and on the trails. It’s an inexpensive traction enhancer and easily rebuildable down the road. A trick that differential rebuilders like Johnson use to get a new ring gar on the carrier is to heat the gear up to about 225 degrees on a hot plate. The hot gear (wear heat-resistant gloves!) dropped right into place when we utilized this method.
G2 instructions call for 60 ft-lb of torque on the ring gear bolts. Matt put a dab of thread locker on each bolt to ensure they stayed secure.
Inspect both the VSS sensor and the tone ring carefully. If in doubt, replace. Our Bronco’s vehicle speed sensor looked a little suspect. Instead of taking a chance on it being bad, we replaced it with a new one.
We were able to reuse the factory tone ring. Johnson put it on a hot plate, then used a race punch to carefully tap it onto the carrier face. The ring has a notch on the inner circle that goes into a corresponding notch on the carrier to keep the ring from slipping.
A trick that helps speed up the 8.8-inch ring-and-pinion swap is to reuse the factory pinion shim because it’s already matched to the axlehousing. The OE shim is usually the correct thickness. If it’s not, the G2 Master Installation Kit includes a full set of pinion shims.
The pinion preload on the 8.8-inch Ford differential was set between 14-19 in-lb with a torque wrench. Ours was spot on at 18 in-lb, thanks to Johnson reusing the factory pinion shim.
The 8.8-inch Ford rear differential is small enough that it was easy to slide back into the housing. Don’t forget to put on the ABS tone ring!
Matt checked the tooth pattern to make sure the ring-and-pinion were aligned properly. You want the pinion teeth pattern to be centered and deep in the ring gear teeth.
It’s a must when rebuilding a diff to have a dial indicator to check backlash. Ford specs call for the 8.8’s backlash to be between 0.011-0.016-inch. Ours was set on the closer tolerance. It’ll open up a couple thousandths as it wears.
Here’s the new differential assembly installed.
We swapped out the old axleshafts with G2’s forged 31-spline versions. The G2’s are considerably stronger than the OE axles.
Reinstalling the C-clips with the new axle repair bearings and Posi was a chore because there’s so little room in which to work. But persistence did get them back into place on the new axleshafts and sucked into the security of the side gears.
Johnson removed the built-in fill plug on our spiffy new gloss-black aluminum G2 Torque differential cover and added synthetic gear lube (treated with a friction additive) to the rearend. The Torque diff cover has a web-reinforced design and special “load bolts” over the carrier bearing caps that reduce the amount of flex, which significantly reduces bearing wear and chipped teeth on the ring-and-pinion. The covers are also equipped with a dipstick and a magnetic drain plug for easy service.
The rebuilt 8.8-inch rearend is lookin’ good. It has the correct gearing and new parts to make it both durable and a good performance upgrade for our 5.0L Bronco.