One two three, four five, six seven, eight nine ten, eleven tweeeeeelllllllve… bolts. OK, so that was just for the Gen Xers out there who can’t only think about pinball right now. The classic GM 12-bolt rear end is the steeped in history, mystery, and fantasy. Let’s break down this venerable, yet elusive, drivetrain component that has long fueled the bench racing argument of which rear axle is the best rear axle.
There are several lauded axles in the world of hot rods, muscle cars, and trucks. The Ford 9-inch, the Chrysler 8 3/4, Dana 60, GM 10 bolt rear end (read this before you start clownin’), and of course, the Chevy 12 bolt. So other than brand loyalty, what is the reason to choose one over the other? In today’s aftermarket, you can get any of these style rear axles in any chassis arrangement, so why the 12-bolt over a 9-inch?
First off, while we mentioned them as a much-loved rear, we are going to drop the GM 10-bolt and 8-3/4 dodge rear end units from this article. While these are EXCEPTIONALLY good rears, their power handling stops at around 500hp (10-bolt), and 750hp (8-3/4), we are focusing on the GM 12-bolt rear axle for the remainder. Even though the Ford 9-inch gets more action in the aftermarket, when comparing stock to stock, the 12-bolt is a little stronger and more efficient, but more on that later.
What Years Did GM Make the 12-Bolt Rear End?
Released in 1964, two versions of the GM 12-bolt axle were used in cars and trucks. Considered the best performance car live-axle rear end from GM, the car 12-bolt gets its name from the number of bolts used to secure the ring gear to the differential, not the number of bolts on the rear cover, though those usually coincide. GM’s car 12-bolt was built for just eight years, from 1964 to 1972, making it extremely hard to find an original housing.
Vehicles built with 12-bolt car rears:
- 1964-1972 A-body (Chevelle, Cutlass, GTO)
- 1964-1972 B-body (Caprice, Impala)
- 1967-1972 F-body (Camaro, Firebird)
- 1964-1972 X-body (Chevy II, Nova)
Trucks and vans got a different version of the 12-bolt, this is called the “truck 12-bolt,” which was built from 1964 through 1987. The truck 12-bolt was the only rear end used in all GM trucks through 1972, until the corporate 10-bolt rear took over. The 12-bolt remained an option through 1987.
Are All GM 12-Bolt Rear Ends the Same?
Not even a little. There were car GM 12-bolt axles and truck versions, and nothing interchanges between the two versions. There are housing differences in each version as well, depending on the year and model they were installed in.
GM 12 Bolt Truck Rear End Details
Truck 12-bolt rears are weaker than the car versions, mainly due to the smaller interior components. The pinion gear shaft is 1.438-inch, smaller than the 1.675-inch of the car unit. The axles have 17 thick splines, and the differential itself is narrower. Identifying a truck 12-bolt is easy, it’s under a truck! The cover has an irregular shape with a raised area for the ring gear. 1963-72 truck housings have a Panhard bar mount near the top of the passenger side axle tube, just to the right of the center section. All truck housings have two lower mounts, either for trailing arms or leaf springs. Trailing arm mounts are located on the bottom of the axle tubes, slightly angled towards the center, while leaf spring pads are perpendicular to the axle tube and mounted at the top of the tubes. Truck rears have larger brakes than car rears. You can build a 12-bolt truck rear for high-performance, as these rears are much easier to find than car versions.
GM 12 Bolt Rear Axle Car Details
The car 12-bolt is nothing less than legendary. These units position the pinion gear higher on the ring gear, where there is less stress and friction, giving the 12-bolt a substantial boost in efficiency over the 9-inch Ford, about three percent better. That’s 3 hp per 100, at 500 hp, you have an effective boost of 15 plus ponies. Add to that the fact that the GM 12-bolt is 30 lbs. lighter, you can see why the 12-bolt is so popular.
Most car 12-bolts were installed in vehicles that used the triangulated 4-link rear suspension, meaning you have a pair of bushing mounts on the bottom and a pair of bushing eyelets astride the top of the center section which run on an outward angle. All GM passenger cars used this style except for the Camaro and Nova, which used leaf springs. The cover is an oval shape with an odd “hat” shape on the top of the cover; it is very distinctive.
Chart GM 12 Bolt Rear End Identification
Casting numbers for the 12-bolt housings are cast on the upper driver-side rear of the center section. For 1964-1969 applications the first character is a letter for the month of the year, A for January, B for February, etc. Followed by the day and the last digit is the year, for example – C287 would be March 28th, 1967. Starting with the 1970 model year the first two characters became digits, 01 for January, 02 for February, etc. Followed by the day and the last digit is the year, for example – 04173 would be April 17th, 1973. On the passenger-side front tube, the axle code is stamped, with a code for 1969 and earlier and a code for 1969 and later builds. 1969 and earlier codes have two letters, followed by a four-digit number, followed by a letter and possibly a shift number (1 for day, 2 for night) and a positraction code. For 1970 and later, these typically feature 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, the next three numbers are the build day from 001 to 365. Sometimes the shift code is stamped (1 for day 2 for night), and if the unit has a positraction, there will be a P stamp.