In this photo, you clearly see the clutch packs with springs, so indeed these are limited-slip differentials. A Yukon aftermarket clutch-type limited-slip differential is on the left; the GM Posi-Traction differential from a 1971 Buick Gran Sport 8.5 10-bolt is on the right. As you can see, the Yukon casting is much thicker and so are the springs.
The C-clips are not the strongest method for retaining the axle shafts; many owners convert the Chevy 10- and 12-bolt axles to a flange type, which retains the axle if it fails. To remove the C-clip, you push the axle in to allow room to snag the C-clip with a pick. Once the C-clip has been removed, the axle slides out of the housing.
The placement of the casting numbers on an 8.2-inch 10-bolt varies by year and model. When you decode these numbers you can conclusively identify your axle.
Engine torque and suspension loads are placed on the rear axle assemblies, which are also subjected to moisture, dirt, and anything the road can throw at it. You may need to clean the rear housing before you can decode the casting numbers. You can simply clean the area around the casting pad, but a power washer and some hot soapy water can work wonders for 40 years of grime.
10-Bolt Housings by the Numbers
Before you rebuild any axle, you should identify which axle you have. Once you have identified the housing, you must order the correct parts for the particular axle. The casting numbers for 10-bolt rear differentials are typically located either on the forward side of the passenger-side axle tube or on the driver’s side. These numbers are approximately 3 inches from the center section.
The two examples at right show you how to decode 10-bolt housings.
1970 axle code: COZ 01 01 G E
| COZ | Ratio |
| 01 | Month |
| 01 | Day of month |
| G | Plant |
| E | Posi-Traction source |
1971+ rear axle code: CB G 112 1 E
| CB | Ratio |
| G | Plant |
| 112 | Day of year |
| 1 | Shift |
| E | Posi-Traction source |
10-Bolt Gears by the Numbers
Gears are also “coded” with their teeth count; dividing the number of ring gear teeth by the number of the pinion gear teeth yields the ratio.
A full range of pinion gears is offered for the Chevy 10- and 12-bolt axle assemblies so you are able to select the correct gear set for your vehicle, application, and setup. These are two pinion gears for the 8.5-inch 10-bolt. The pinion on the left is part of a 4.11:1 gear set; the one on the right is a 3.08:1 pinion. You can see the dramatic difference in not only teeth but in overall diameter.
The tooth count is stamped on the head of each pinion for both the pinion and the ring gear. As you can see, 13 is the hypoid gear count for the pinion and 40 is the ring gear count. Pinion gears and ring gears are not interchangeable because they are designed for the specific (correct) mesh. Therefore, the specified pinion and ring gears must be used together.
Limited-Slip versus Open Differentials
As you may know, GM’s product name for a limited-slip differential is Posi-Traction. Most GM muscle car owners choose to install or use an axle assembly with Posi-Traction because it provides superior traction and performance compared to an open differential. A limited-slip differential senses wheel slip and directs torque to the wheel that is not slipping, so acceleration is optimized.
Two methods are used to determine if an axle has a limited-slip differential. A strong but not definitive indication that axle assembly has (or once had) Posi-Traction from the factory is a metal tag on one of the rear cover bolts that reads, “Use Limited Slip Lube Only.”
However, a hands-on mechanical test of the differential is the only certain way to determine if it is indeed a limited-slip. First, you must raise the rear of the vehicle and place the vehicle on jackstands. Set the transmission in neutral. Grab one wheel and spin it while having a helper watch the other wheel. A correctly functioning limited-slip differential spins both tires in the same direction. An open differential spins the other wheel in the opposite direction.
Although this hands-on test provides a good indication, it is not conclusive because a badly abused differential with worn clutches can function like an open carrier.
When it comes to GM muscle cars and sports cars, the 12-bolt axle has been the top high-performance axle assembly for decades. Compared to the Ford 9-inch, the 12-bolt positions the pinion gear higher on the ring gear. This reduces the load on the pinion, resulting in less parasitic loss from the friction and load.
The 12-bolt was introduced in 1964 and installed in cars and trucks until 1972. From 1972-on, General Motors installed its 10-bolt in cars and it remained an option for trucks until 1987.
This 1967 Chevy truck used a trailing-arm design with coil and leaf springs. The half-leaf spring (left) serves as an overload spring for heavy loads or trailering.
Unlike the various 10-bolts, the 12-bolt axle assembly has different components for cars and trucks. The passenger car 12-bolt has an oval-shaped differential cover, and it measures 10
Trucks have a smaller inner pinion shaft (1.438 inches versus 1.675 inches) and bearing, and the pinion rides lower on the ring gear. In addition, the truck 12-bolt has an irregular shape. The early truck 12-bolts had large axle splines with only 12 splines. The differential carriers are also narrower than on the passenger car units, and they do not interchange. That does not mean that the truck units are not capable of performance builds because aftermarket 30-spline carriers and axles are available.
General Motors installed different axles for different applications. Axles for high-performance or heavy-duty applications commonly used higher spline-count axles while common passenger car axles use lower spline counts. The top axle is an 8.5-inch 10-bolt with 30 splines; the bottom axle is an 8.5-inch with 28 splines. Note the