How To Rebuild an Eaton Clutch-type Limited Slip Differential (2023)

Jefferson BryantWriter, Photographer

General Motors has several names for its LSD, or Limited Slip Differential, unit. Commonly referred to as Posi-traction, the exact same clutch-based LSD is also known as Controlled Traction, Saf-T-Grip, and Positive Traction, but the unit is the same. It is a clutch-type LSD, which means there is a stack of clutch discs that pack together to provide grip.

In the standard pack, there are nine clutches per side, five locking discs with tabs on the outer side that lock into the case, and four discs that are splined to lock onto the side gears. Input torque acts on the meshing teeth of the spider gears, forcing them apart and pressuring the clutch packs. This provides a positive coupling of the axles. Because the unit uses clutches, eventually they wear out, essentially becoming an open-type differential.

Most of the factory clutch-type LSD units are based on the Eaton LSD, meaning the parts interchange across several brands and sizes. The clutches for GM 10 (8.2- and 8.5-inch) and both car and truck 12-bolt LSD units are the exact same as the Ford 9-inch and 8.8, and the Chrysler 8 , making the parts readily available. There are a few options four to be exact.

Steel Clutch Packs
Most stock LSD units use an 18-disc clutch pack, with nine discs per side. These solid discs use high-quality steel with cross-hatched lines on both sides. This is the friction surface, which is metal-on-metal. These do wear out, but it takes a lot of time for that to occur. Unless the steel discs are severely worn, the clutch pack can be shimmed to increase pressure on the discs, regaining the limited slip action. These discs are very strong and don't break, but they tend to chatter, especially with heavier center springs.

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The GM service kit uses an 18-disc pack with a series of slots cut around the perimeter of the clutch discs. The slots help reduce chatter, but end up weakening the disc, leading to breakage. They still chatter when used with 400 and 800-pound springs, which is why most builders prefer to use solid discs.

For race cars, the Eaton 22-disc clutch pack is available, and works well in extreme conditions, until the locking tabs on the intermediate discs wear out. However, these discs chatter with all spring combinations and require frequent service.

Eaton also produces a carbon-fiber clutch kit for LSD units. The 14-disc pack is complete with seven discs per side, and uses a more traditional fiber clutch and steel plate configuration. Each of the three clutch discs has a carbon fiber pad on either side that engages the smooth steel surface of the intermediate discs. In this pack, the clutches have tabs that lock into the case on the side, and the intermediate discs lock onto the side gear. The carbon fiber material provides excellent grip without chattering when used with 400-pound springs or less.

Spring Rates
There are four spring rates for Eaton clutch-type LSD units—200, 300, 400, and 800 pounds representing preload on the clutches. These spring kits are available from Eaton or GM. For most street performance vehicles, the 400-pound springs are best, as the 800-pound springs are very tight and will produce more chatter and less slip in the corners. All Eaton posi units come from the factory with 400-pound springs.

The carrier must be removed from the vehicle to rebuild it. In most cases, the cross-shaft will already have been removed to get the carrier out of the housing. If not, such as in vehicles with bolt-in axles, remove the cross-shaft.

The remaining process should be done with the case cradled in a bench vise. The ring gear pad works well; place it in the vise and clamp it down, with the large hole in the case facing up. The springs and plates must be removed first. Using a prybar or screw driver, gently pry on the plates to slide them out of the housing. As the assembly begins to come out, use a clamp to keep tension on the springs, otherwise they will go flying everywhere. You can release the tension slowly once the assembly is fully removed from the housing, or keep the assembly compressed.

The spider and side gears work together in mesh, and need to be spun to remove them. Rotate the side gears until the spider gears reach the large hole and remove them. Then, rotate the side gears twice, as the one spider will work its way 180 degrees from the hole. With the spider gears out, the side gears slide right out of the case. Be mindful of the locking clip that holds the discs in the slot. There are two clips per side. Most rebuild kits include these, but if you are just shimming the posi, you will need to keep them for future use.

At this point, all the main components should be out of the case and on the bench. Remove the clutch packs from the side gears, paying attention to how they are positioned on the gear, including the shims between the pack and the side gear. Keep this arrangement side-to-side.

Checking The Parts
Start off by inspecting the gears. They should be smooth, without any chips, missing teeth, or rough patches. The spider gears should have smooth, shiny surfaces where they ride in the case, and should not be pitted or discolored. The side gears should be clean and free from chips, scarring, or missing teeth. Be sure to inspect the inner splines of the axles and outer splines of the clutches for damage

The clutches should be intact, free of breaks, cracks, or discoloration. If e reusing the clutches, make sure the crosshatches are uniform and not significantly worn. If replacing the clutches, this inspection is not critical. Look at the case for obvious signs of wear and replace any parts that are suspect.

Assembling The Clutch Pack
The clutches go together in a specific sequence. From the inside of the side gear: locking disc, spline disc, locking disc, spline disc, and so on until the pack is fully stacked and topped off with the original shims. The number of discs will vary by the style of clutch. For carbon fiber clutches, the clutch material is on the locking disc, with the splined disc between. The stack sequence is the same. The clutches are wet-type, meaning they need to be lubricated. Apply new gear oil to each disc, including the inner side gear surface, before stacking the pack.

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Installing The Gears
Once the clutch pack is assembled, the parts can go back into the case. The side gears are fairly simple, but the locking tabs can fall off easily, so dab a bit of grease on the locking clip to hold them in place. The spider gears are tricky; compress the packs on both sides and spin the side gears at the same time. There is not much room for two sets of hands, so improvise, here. If you have a spreader clamp, that would work well. For our purposes, we used a bolt with a coupling nut to spread the load across the gears.

Place the spider gear in the housing and roll the gears until it is at the 180-degree location below the large opening. Reset the spreader device, and place the other spider gear into the housing. If you have it just right, the two gears should line up with the cross-shaft holes. If you are one tooth off, you must start over. Once the gears are aligned, the spring pack is reinstalled along with the cross-shaft.

Testing The Differential
Using the method most builders use, check the action of the differential by using the axle shaft to engage one side gear and spin it or clamp the axle into the vise (if the bench and vise that will allow it), and rotate the housing. If the housing spins easily, add more shims to the clutch pack, adding and equal number of shims per side. If it is tight, and requires a lot of effort to spin it, it should be good.

The spider gear clearance can also change how the unit locks up. Shims are available to tighten up the spider gears, in either steel or brass. If your unit had shims between the case and the spider gear, reuse them. If the gears are not tight to the side gears, use a thicker shim. The case shown here did not have any shims, and the gears were nice and tight. This is where experience and feel come into play. The goal is to have a nice, tight differential. If there is any play or it spins easily, you need to add shims.

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1. 1950's GM Eaton Limited Slip Differential Explanation and Assembly
2. Plated/Clutch, Torsen, and Open Differentials Compared with Track Test
3. How Clutch Type Limited Slip Differentials Work
4. How to fix a limited slip differential
5. Working of Limited Slip Differential
6. Rebuilding your 12 bolt eaton posi. How to.
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