Eaton 912A556 Detroit Truetrac Limited Slip Differential for GM 8.5-in 10-Bolt

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Product Specification:

SKU:EAT912A556

MPN: 912A556

Weight: 26.4 lb

Inventory: 10

Type: Differentials

Vehicle Fitment

1971-1989 GM Passenger Cars with 10-Bolt 8.5-inch rear axles
1972-1989 GM Pickup/Half-Ton Trucks with 10-Bolt / 8.5-inch rear axle
1978-1989 GM Three-Quarter-Ton Vans with 10-Bolt 8.5-inch rear
Various GM models from early 1970s through late 1980s using 10-Bolt 8.5-inch C-Clip style axles

Fitment Notes

Requires GM 8.5-inch (or compatible 8.6-inch) 10-bolt rear axle housing.
Must use axle shafts with **28 splines** and approx. **1.20 in** shaft diameter.
Ring & pinion gear ratio must be **2.73 or numerically higher** (e.g. 3.08, 3.55, etc.).
Special carrier bearings required depending on axle bearing bore size: small bearing-bore (≈2.89 in race/O.D.) or large bearing bore (≈3.06 in) are options.

Features

Helical-gear (Detroit Truetrac) style limited slip differential - uses planetary helical gears to transfer torque.
Smooth, quiet automatic engagement - works without driver intervention.
No wearable clutches or friction discs to replace.
Designed for frequent load, towing, or performance use.
Maintenance-free operation when proper gear oil is used.
Compatibility with a variety of gear ratios for flexibility in vehicle setup.
Durable steel construction for long life and resistance to axle loads.
Weight is ~26.17 lb - robust yet manageable for installation in many rearends.

Specifications

Part Number: 912A556
Axle Type / Housing: GM 8.5-inch 10-bolt rear axle
Axle Shaft Spline Count: 28 splines
Axle Shaft Diameter: ~1.20 in (≈30.48 mm)
Ring Gear Size: 8.5-inch; fits GM 8.5-/8.6-inch designs
Gear Ratio Compatibility: 2.73 and numerically higher ratios (2.73:1 up)
Product Weight: ~26.17 lb
Cover Bolt Count: 10 bolts

Included Components

1x Eaton Detroit Truetrac Differential Assembly (Part # 912A556)

Product Description

The Eaton 912A556 Detroit Truetrac is a high-quality limited-slip differential designed for GM 8.5-inch 10-bolt rear axles. It leverages Eaton's helical gear system to provide automatic torque biasing: when one wheel loses traction, power is smoothly and quietly redirected to the other wheel without any clutches or driver input. This results in superior grip in slippery or challenging driving conditions, while maintaining car-friendly behavior in everyday driving.

Ideal for GM cars, pickups, and vans from the early 1970s through the late 1980s, especially those with 28-spline rear axles and gear ratios of 2.73:1 or steeper. Before installation, verify your ring & pinion ratio, axle spline count, and bearing race bore size, since those affect compatibility. The proper bearings are required depending on whether the axle has a small or large bearing bore. This part is robust and suitable for performance, restoration, street & light off-road use.