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Engines & Drivetrain
A 12-bolt behind an overdrive gets some low-end punch
May 3, 2005
Corona, CA 92879
Randy's Ring & Pinion
Everett, WA 98204
Detroit Locker/Tractech USA Headquarters
Madison Heights, MI 48071
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Gary Maib of Diff Works in Riverside, California, was kind enough to let us look over his shoulder while he completed the gear and axle change. The first step is to pop the cover and drain the gear lube.
Once drained he scraped off the remainder of the gasket and cleaned the gasket surface of the housing.
This particular vehicle is equipped with rear discs. In order to pull the axles, the calipers and rotors will have to be removed. On drum-equipped vehicles the drums and backing plate assemblies will have to be removed, as well.
GM rear axles are held in place with C-clips. The OEM differential uses a pin that slides between the two axle ends, which exerts a bit of outward pressure on the axles thus holding the C-clips in place. The pin itself is held in place with a locking bolt that must be removed before the pin will come out.
Gary uses a magnet to snare and remove the C-clip axle retainers. Once both are removed the axles can be slid out of the housing.
If the axles were to be re-used they could just be pulled out, as shown, and left sitting in the housing while the work takes place--since he'll be replacing the OEM axles for a pair of Yukon items they were fully removed from the housing and set aside.
After removing the bearing caps (pay attention as they're distinctive left- and right-hand items) the differential is coaxed out of its nest. It often takes a bit of muscle to get the differential to "pop" though I've seen some virtually fall out, as well.
Once the differential is out of the housing you can then remove the pinion yoke and then the shaft.
Like the differential, the pinion often needs a bit of coaxing to come loose.
With the guts removed the next step is to drive out the pinion bearing races. Using a punch, and a fair amount of finesse, drive the forward race out the front and the rear race out the back.
Now that you've got an empty housing, it's time to give it a good cleaning. This not only removes residue from the original gear lube but washes out any foreign matter, as well.
A neat trick for installing the new pinion races--the front one, at least--is to use the old one to drive in and seat the new one. The rear, or inner race, can be installed using the punch used to drive out the old one--again, use some finesse.
For maximum performance, Gary chose a Truetrac Posi differential and a Yukon ring-and-pinion set. The ring gear is prepped for attachment to the differential by scuffing the mating surface with 80-grit emery cloth to remove any rust preventative or burrs that'd interfere with a perfect mating with the differential.
The ring gear is then attached to the Truetrac. The bolts are treated to an application of thread locking compound and then torqued to specification.
The differential bearings are then pressed into place on the unit. The Truetrac differential is unique in the fact that it performs like a conventional differential, that is, until there's a loss of traction. Only then will the power transfer occur--when it's needed. The big benefit is that the Truetrac increases traction but doesn't effect steering or wear prematurely, which is a common problem with limited-slip differentials that use clutches and springs.
Here, Gary is shown adding the appropriate shim to the pinion prior to pressing the pinion bearing onto the pinion shaft and readying it for installation into the housing. Most new ring-and-pinion sets are pre-run and marked on the pinion shaft with its proper depth setting known as the "checking distance." This dimension is from the face of the pinion to the axle centerline.
Pinion depth is adjustable by adding or subtracting shims, and a good rule of thumb is to stay within +/- .002 of the checking distance. Once loaded with a new crush sleeve and shim pack the setting determines the gear load pattern.
The pinion is slipped into place, the yoke slid onto the pinion shaft, and the nut started and torqued to specification compressing the crush sleeve and setting the bearing preload (as a rule of thumb, 25 lb/in of rotating torque for new bearings). It's is also a good idea to give the pinion assembly a healthy tap both front and rear. The slight jolt serves to assure that the pinion assembly is truly set in its races.
The Truetrac differential (with its bearing races in place) is then set into the rearend housing journals. It should be a relatively snug fit. Once in place it's time to check backlash. Again, as a rule of thumb, most gearsets are developed to run at about .008- to .012-inch backlash for street gears. Adjustments for backlash are provided by shim packs behind the bearing cups. When the backlash is within specification the bearing caps are installed (pay attention as there is a definite left and right cap) and torqued to the proper rate.
At this point it's about time to button up the differential and check the gear tooth-contact pattern. Gary's third arm comes in pretty handy for speeding things up when the clock is ticking--just kidding folks.
The teeth are painted with gear marking compound and the ring gear is rotated several revolutions. A tooth-contact pattern will then appear and should be similar to the pattern shown in the installation instructions supplied with the gearset.
If the pattern is not in the approximate position shown in the instructions, reset the pinion depth and backlash to correct the pattern. Most times the shims must be changed in .003-inch increments to notice a pattern change. If a pattern is heavy toe (top), subtract shims. If it's heavy heal (bottom), add shims.
The new Yukon axles are then inserted in the tubes. They're held in place by C-clips that fit into a machined groove in the axle. In this case you can access them through the rectangular opening in the Truetrac differential.
The Truetrac spacer block and its retaining bolt are then inserted between the axle stubs, the block pushes the axles outward applying pressure that holds the C-clips in place.
The final steps are to replace the gasket and cover, fill the rearend with a high-quality gear lube, and reinstall the brake assemblies. The gear change and Truetrac in combination with the vehicles existing Gear Vendors overdrive unit should really make this bad boy a fun ride.
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Changing gears and upgrading to a positraction rearend
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