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Rearend Redux

Posted in How To: Transmission Drivetrain on August 14, 2017
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One of the best scenes in “My Cousin Vinny” was when Mona Lisa Vito, played by the adorable Marisa Tomei, is on the witness stand as an automotive expert. The case hinged on a set of tire marks. When asked how the marks were made and what a limited slip differential is, she said “A limited slip differential distributes power equally to both the right and left tires,” and then aptly explained, in a heavy New York accent, what an open differential was with “Anyone who’s been stuck in the mud in Alabama knows you step on the gas; one tire spins and the other tire does nothing.” Funny and accurate.

We may not deal with Alabama mud too often, but rocky hills or a deep sandwash are all too familiar of terrain and having the ability to get out of that sand and back to terra firma requires traction. Most vehicles come stock equipped with an open style differential. They work fine on the street as they allow the two axles to work independently from one another, like what’s needed to go around corners.

The Eaton Trutrac is a helical-gear style limited slip differential that uses internal gearing to maximize wheel traction.

But, they can actually make things worse in the dirt as they have a bad habit of sending all the power to the one tire that has the least traction, thus causing that tire to spin uncontrollably and possibly dig a hole that may be tough to get out of. To combat this and give you all the traction that you need, there are things such as lockers and limited slip differentials.

For the absolute in both tires rotating at the same rate, there are lockers. They are either air or electrically operated and are great for hard core 4 wheeling where putting the power equally to both wheels is needed for slow climbs over big rocks. But for us who drive trucks on trails and in the desert, they may not be the best call.

That would probably be an LSD, or limited slip differential. There are two main designs of LSD’s as one uses helical gears and the other uses springs and clutch style friction plates. Eaton makes both styles of LSD’s, and they have been around as long as autos themselves. Though now a huge international conglomerate with ties from aerospace to electrical relays, the first product they offered was a patented internal gear truck axle way back in 1911 as Torbensen Gear and Axle. Now, they offer everything from the iconic “Detroit Locker” to the E-Locker to the subject of this install, the Trutrac.

The forged G2 4:56 gear set is made from heat-treated 8620 steel and machined to OE specifications.

According to Eaton, “The Truetrac operates as a standard or open differential under normal driving conditions, allowing one wheel to spin faster or slower as necessary. When a wheel encounters a loss of traction or the terrain changes, the gear separation forces take effect and transfer torque to the high-traction wheel. The helical-shaped gears mesh with increasing force until wheel spin is slowed or completely stopped. When the vehicle exits the low traction situation, the differential resumes normal operation.”

The owner of this truck likes to drive off-road and the open differential that his Ford F150 had was ok to a point, but says that even with 4 wheel drive there were times when we was worried about losing traction when he needed it most. These were either sandy or muddy conditions or when going up a steep hill. He said he went with the Trutrac because of the towing he does and felt the helical gears were the stronger option over the spring and plate design.

He may have let it go if not for the fact that he really needed to change the gearing. Having swapped out stock tires for some 35-inch BFG KO2’s, the 3:73 gears were now not close to being right. He stated that the engine was barely turning 1800rpm at 75mph (not that we condone going over the posted speed limit…). Their wrongness was especially evident whenever the owner tried to pull his fully loaded toy hauler up any hills. He was afraid he was going to hurt the engine or trans. A change to 4:56 gears was required, so he figured he do it all and add an LSD while things were apart.

G2 also offers this bearing and shim kit to install their gear set. It’s a must have for the job and is designed to work exactly with the G2 gears.

G2 Axle and Gear also offers a lot of products to their customers. They produce everything from gear sets to complete bolt in-ready rearends. They also had the appropriate 4:56 gear set to fit the 8.8 Ford rearend in this F150. Forged from 8620 steel, heat-treated and machined to OE specs, these gears will put the powerband back where it needs to be. Not known for great low-end torque, the 5.4L V8 will at least have a chance with the 4:56’s, and can only benefit from the addition of the Trutrac. Some think that a Trutrac in back and an on-demand locker up front is the ultimate off-road setup for serious chasing and overlanding with a 4x4 pick-up. We think that setup could put a hurt on winch builders everywhere.

This install happened at the Riverside CA. 4-Wheel Parts store. Both the front and rear differentials received the G2 4:56 gears though a Trutrac was only installed into the rear one. Speaking of installing things, know that each end is about a 4-hour job plus parts, so expect a bit of sticker shock. Also know that the gears must be broken in reasonably slowly. Short trips of less than 50-mile duration is recommended for the first 200-500 miles, and no heavy towing for about the first 1000-miles.

Follow along and see how easy it isn’t to get a truck ready to take on multiple roles. From tow rig to overland explorer, a 4x4 truck can do it all; if it has the right pieces, that is.

The truck was already equipped with a Mag-Hytec differential cover, so draining the fluid was as easy as pulling the plug and letting it drain.
Since the axles need to be removed, the brake assemblies are pulled.
The driveshaft too needs to be disconnected and pulled out of the way.
A punch is used to put markings on the bearing caps (It’s normally one dimple on the upper left and two on the upper right), as they need to be put back in exactly as they were removed.
A “differential pinion shaft” holds the stock pinion gears in place, and it in turn is held in place with the “differential pinion shaft lock bolt.” Care is taken as when the bolt and then the pin are removed as the gears will fall out. With the gears out of the way, the axle clips are removed and the axles pulled.
Care is taken when removing the stock open differential so that the side shims don’t get mixed up. They will be used as a baseline reading for the new shims.
There is an “exciter ring” that needs to be reused on the new Trutrac unit. It’s sandwiched between the gears and the case. This exciter ring is how your anti-lock brakes are activated.
A good spray of brake cleaner is applied to the new G2 gears in preparation of installing them.
Though the bolts already have a thread-locking compound on them as they come from G2, a little more Loctite couldn’t hurt.
The G2 ring gear is installed onto the Trutrac unit and the bolts are torqued to as per G2’s specs.
The bearings are then pressed on using a hydraulic press.
The press is then used to install the bearing onto the G2 pinion.
Getting the old pinion out takes muscle, so a pneumatic gun with a pointed bit is used to drive the pinion out.
A bar is used to remove the old seal and the pointed bit equipped pneumatic gun is again used to drive out the old bearing race.
A specialized tool is used to seat the new race. There is a telltale hollow sound made when the new race is bottomed out.
A little gear grease is applied to the pinion bearing. Notice that the crush sleeve is not installed at this point.
A pneumatic gun is used to install the pinion nut. The pinion needs to be snug in that it needs to spin but not have any in-out play.
The stock shims were used for this trial fitting.
The caps are installed and snugged down.
A dial indicator is used to set the backlash. As per G2 specs, the backlash should be .011-.016.
It was determined that there a little too much movement, so additional shims were needed.
That meant the Trutrac case was tight. Since the shims are steel, a piece of aluminum was used to gently tap the shims into place.
After again checking the backlash and determining it was within specs, it’s time to check the pinion depth. That’s done with painting the ring gear teeth with a special paint and rolling the assembly around a few times. The ideal is to have the ring and pinion gears mating up near the center.
The mating of the ring and pinion gears leaves a pattern on the paint. This pattern is very close to perfect, so the Trutrac and pinion assemble is removed…again.
After greasing the surfaces, the new seal is tapped into place.
The crush sleeve is installed onto the pinion shaft and the pinion is put back into position.
After hitting the threads with Loctite, the pinion nut is installed. G2 specs say that for new bearings there should be approx. 14-19 in. lbs. of pre-load on the pinion. Too much pre-load and the bearings could be damaged, too little and the pinion could wobble enough to damage the ring gear, bearings or races.
The Trutrac/G2 ring gear assembly is installed and the cap bolts torqued down to 60 ft. lb.
The axles are installed and the retaining clips are installed. The axles are pulled back out slightly so the clips are seated.
The spacer is installed. This spacer keeps the axles correctly spaced and seated.
A hardened steel cap goes on next. It will be secured in place with a C-clip.
The driveshaft is bolted up.
Brake clean is used to clean the surfaces of the cover and the housing.
The Mag-Hytec cover goes back on and a rotating pattern used when tightening the Loctite covered bolts.
Lucas Oil Synthetic 75W-140 gear oil will be the perfect compliment for this now ready to roll rearend.
It takes nearly four bottles of the Lucas product to fill the Mag-Hytec cover to its proper capacity.


Van Nuys, CA 91402
Cleveland, OH 44114
Lucas Oil
Corona, CA 92880
G2 Axle & Gear
Compton, CA 90220
4 Wheel Parts
Compton, CA 90220

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