The truck 12-bolt axles are much more affordable than the car units because they are more plentiful but these units have fewer splines so they are not as strong as the axle in the car assemblies. In addition, the trucks typically have larger axles and brakes.
Most passenger car 12-bolts used a four-bar trailing arm mounting system, with the exception of the Camaro and Nova, which used leaf springs. GM trucks from 1961 through 1967 used a two-bar trailing arm mount, while the 1968-up trucks used leaf springs. There is some crossover on the trucks, as some earlier trucks had leaves and some later trucks had the trailing arms.
All GM 12-bolts use C-clip–style axles. Aftermarket 12-bolt housings are based on the passenger car design.
General Motors used several different suspension designs in their passenger cars throughout the 1960s and 1970s. This 8.5-inch 10-bolt came from a 1971 Buick GS. The large bushings at the top of the differential housing connect to the triangulated four-bar trailing arm system that the Buick used. It is more difficult to swap these housings from car to car if they do not share the same suspension design.
Camaros, Novas, and 1968 and later trucks used leaf springs like these. The axle may be over or under the leaf, depending on the application.
C2 and C3 Corvettes (built from 1963 to 1981) used a non-standard 10-bolt design. They used an independent rear suspension with transverse leaf springs. As a result, these cars use a specialized axle housing for this suspension, and it’s not easily upgraded. You need to machine the housing to accept a 12-bolt carrier, which also requires custom axles. Essentially, the housing is machined to clear the larger gears and carrier, and it’s not a job for the novice.
The Chevy 12-bolt axle assemblies for passenger cars feature an oval cover with a diagonal indentation. This is a 1969 Chevelle 12-bolt housing.
12-Bolt Carriers
The 12-bolt carriers also use the same series-specific system as do the 10-bolts; each carrier only works with certain gear sizes. The types are 2-, 3-, and 4-Series. The 2-Series is by far the most common.
12-Bolt Housings by the Numbers
The casting numbers for the 12-bolt housings are typically found on the upper rear of the driver’s side of the center section. The casting numbers are simple to decode.
The first letter is the month of the year; A is January, B is February, and so on. The next digit is the day it was built, and the last digit is the year it was built. For example, a 12-bolt axle that was built on March 28, 1967, is C287.
On the passenger-side front tube, the stamped axle code designates either 1969-and-earlier units or 1969-and-later builds. The 1969-and-earlier codes have two letters, then a four-digit number, followed by a letter, and possibly a shift number, for which 1 is the day shift and 2 is the night shift.
And finally, a Posi-Traction number was used.
For 1969 and later, the code typically features six to eight digits, including three letters, three numbers, and sometimes an additional number and letter. The first two letters indicate the gear-ratio code, the third letter notes the build plant, and three numbers designate the build day from 001 to 365. Sometimes the shift code is stamped, and if the unit has a Posi-Traction, you see a P stamp.
Truck 12-bolts have an irregular cover with a ring gear pocket. This example is a 1967 Chevy C10. The truck housings are not as durable as the passenger car housings due to a narrower carrier and a smaller inner pinion bearing.
SUSPENSION TYPES AND DIFFERENTIAL HOUSINGS
General Motors has been using variations of the 10- and 12-bolt differential since the mid-1960s so many different housings are in use. The basic 10-bolt has been used in every type of vehicle, for both rear and front (four-wheel drive) applications. The 12-bolt passenger car design was relatively short lived, with an eight-year run, but the truck version ran through the late 1980s, so many vehicles use those.
GM differential housings have used several types of mounting styles, including leaf springs, coil springs with trailing arms, coil springs with a triangulated four-link, and independent rear suspension (IRS). Front differentials are either leaf springs or independent front suspension (IFS). Some housings can be converted to another suspension system while others cannot; it just depends on the design of the suspension, particularly in the case of four-link rear systems. You can adapt a four-link housing to a leaf spring car, but it requires so much fabrication that it is not practical to install a leaf spring housing to a four-link.
To start the disassembly of the differential, first you remove the center cover. You can clearly see the ring gear and carrier, which contains the limited-slip clutch pack. In a typical high-mileage rebuild, you need to replace most of the bearings, gaskets, and seals.
A leaf spring mounting system uses a pair of arched steel or fiberglass leaf packs to suspend the rear housing from the frame. The leaf packs are typically made up of multiple leaves, but in some cases, these are single-leaf springs. The leaves use two mounting points to the chassis in the front and rear of the housing itself.
The housing may be mounted above or below the leaf springs. The housing is below the springs in most trucks for added ride height, and is called overslung. The housing is typically underslung, or mounted below the spring pack, in most passenger cars, and lowers the ride height.
The leaf spring rear suspension is fitted to Camaros, Novas, trucks, and several other vehicles. This leaf spring Chevy truck has an overslung housing (the housing is positioned underneath the springs).
Older pre-1968 Chevy and GMC trucks