Eaton Detroit Truetrac Differential 28 Spline 1.20in Axle Shaft Diameter 2.73 & Up Ratio Rear 8.5in
A 1970s or 1980s GM car or truck with the 8.5 inch 10-bolt rear that leaves one black stripe on launch, or a Cutlass, Monte Carlo SS, Regal or C10 pickup whose tired factory posi has stopped doing anything, is exactly what the Eaton Detroit Truetrac 912A556 is for. It is a gear-type limited slip carrier for the GM 8.5 with 28-spline, 1.20 inch axle shafts and a 2.73 or numerically higher ring gear ratio.
The factory open carrier splits torque evenly, so the tire with the least grip sets the limit and the other one just sits there. The Truetrac replaces that carrier with a steel case holding helical pinion gears in pockets. Under load the gear thrust pushes those pinions into the case walls, and the resulting friction biases torque toward the tire that still has traction. There are no clutch packs or springs to wear, so it does not chatter in tight parking lot turns and it does not lose bite over the years the way the old clutch posi units do. It works on and off throttle and in reverse. Its one limit: if a tire is completely off the ground or on glare ice it has nothing to push against, so it behaves more like an open diff in that moment.
The 8.5 has a 10-bolt cover just like the smaller 7.5 and 8.2, so the cover bolt count alone does not prove what you have. Measure the ring gear or check the axle code stamped on the right axle tube. Next, pull an axle and count splines: this part is the 28-spline version. Later 8.5 axles and the 8.6 commonly use 30 splines, and those need a different carrier. Our GM 8.5 10-bolt problems guide walks through the noises to rule out first, and the 10-bolt vs 12-bolt guide helps if you are not sure which axle you have.
Eaton's product selector lists 912A556 for the rear GM 10-bolt C-clip axle in 1971-1989 Chevrolet, Pontiac, Buick and Oldsmobile passenger cars, 1972-1989 GM half-ton trucks, and 1978-1989 three-quarter-ton vans, all with 2.73 and up gearing. If your ring gear is 2.56 or lower, or your shafts are 30-spline, this is the wrong unit. Because axles get swapped constantly on these vehicles, send us the year, VIN and spline count if you have any doubt. For a 30-spline 8.5 there is the Eaton Posi 30-spline 8.5 carrier, and replacement 28-spline 8.5 passenger car axles are available if a shaft is worn at the bearing. Other axle parts are in our hard parts section.
| Brand | Eaton |
| Part number | 912A556 |
| Fits | Eaton Detroit Truetrac Differential 28 Spline 1.20in Axle Shaft Diameter 2.73 & Up Ratio Rear 8.5in |
| Type | Differentials |
| Condition | New |
| Ships | Same business day (in stock), from the USA |
| Warranty | 1-year |
Eaton lists 912A556 for 1971-1989 GM passenger cars with the 8.5 10-bolt, so yes if the axle is the 8.5 with 28-spline shafts and 2.73 or higher gears. Confirm the spline count before ordering.
No. This carrier only accepts 28-spline, 1.20 inch shafts. A 30-spline 8.5 needs a 30-spline carrier.
Yes. Eaton requires bearing LM102949 and race LM102911 on small-bore GM 8.5 housings, which is what every 1971-1989 application uses.
No. It is a helical gear unit with no clutches, so regular GL-5 gear oil is all it needs.
Not with this unit. Eaton makes it for 2.73 and numerically higher ratios only; lower ratios sit on a different carrier break and need a different part number.
Carrier and pinion bearings, the pinion seal, crush sleeve and pinion nut, ring gear bolts, and the cover gasket. A master bearing kit covers most of it.
