It wasn’t that long ago when we put together an identification guide that distinguished the differences between various General Motors 10-bolt rearends. To say it was well received would be an understatement. For that reason, we decided that we should also put together a 12-bolt rearend ID guide for those of you who might be looking for this information as well.
When it comes to GM muscle car purists – and enthusiasts searching for the ultimate Chevrolet rearend – the 12-bolt rearend is considered the top of the heap in regards to a high-performance axle assembly. Some enthusiasts feel the 9-inch is a comparable replacement – and an acceptable swap into a Chevrolet muscle car – but when compared to the 9-inch, the 12-bolt has a definite advantage. The 12-bolt positions the pinion-gear higher on the ring gear, which reduces the load on the pinion. This results in less parasitic loss from the friction caused as the gears mesh.
From The Beginning
The 12-bolt was introduced to performance buyers in 1964. While it was originally designed for use in factory-built performance-rated cars, it was installed in both cars and trucks until 1972. After 1972, General Motors was only installing its 10-bolt rearend in cars, but the 12-bolt remained an option for trucks until 1987.
When looking at a 12-bolt rearend, it is important to know that 12-bolt axle assemblies use different components for cars and trucks. This means parts are not interchangeable, which can come back to bite you later if you buy the wrong assembly. For instance, the truck 12-bolt uses a smaller inner pinion-bearing (1.438 inches versus 1.675 inches).
Also, the offset for the ring gear is different. If you are wanting to use a car’s ring gear in a truck differential, it will require some creative shimming to get the backlash set right. Another thing to consider is car axles will not work in a truck housing. This is because not only is the wheel bolt-pattern different, but they use different axle bearings as well.
Trucks first started using 12-bolt rearends in the 1965 G-series vans and C-series pickups. They were used until the end of production in 1987. Blazers from 1969 through 1981 also used this differential (four-wheel-drive Blazers had it installed on the rear only).
At a quick glance, you can also tell when you’re looking at a truck 12-bolt, because the rearend cover has an irregular shape. Early truck 12-bolts had axles with 12 large axle splines. The differential carriers are also narrower than those in passenger-car units. They are not interchangeable. That doesn’t mean the truck rearend is not capable of receiving performance upgrades. To the contrary, aftermarket 30-spline differentials and axles are available.
Most GM trucks from 1961 through 1972 used a rear suspension consisting of two trailing-arms with coil springs. There is an anomaly in that statement though, as leaf springs were an available option. In 1973, Chevrolet pickups began using leaf springs as standard equipment.
Although not technically how it got its descriptive name, the easiest way to identify a 12-bolt rearend is obviously by the 12 bolts holding the rear cover in place. In actuality, the term 12-bolt is due to the 12 bolts holding the ring gear to the differential.
Just like a 10-bolt rearend, there should be stamped numbers on the axle tube – on the passenger’s side. The numbers should let you know what gear-ratio came from the factory, the date of production, where the unit was assembled, and whether the carrier is Positraction or not (see charts).
The 12-bolt was used in many vehicles, so rearend mounting is something you need to keep in mind. Full-size passenger cars used coil springs with four trailing arms. All trailing arm mounts are located on the axle tubes, and the perches for the coil springs are found on the lower trailing arms. This means there will be no spring perches on the rearend housing.
The Chevelle and Monte Carlo also used four trailing arms, but the arms mounted differently. In these applications, the upper trailing arms are mounted on top of the differential, not the axle tubes. The coil springs also mounted to the axle tubes. Finally, Camaro and Nova rear axles used leaf springs. The rearend cover on passenger car 12-bolt rearends is oval, measuring 10-15/16-inches wide by 10-5/8-inches tall. It also has an upside-down, V-shaped protrusion that is there to direct oil to the carrier bearings.
When looking at a swap meet or salvage yard rearend, keep in mind that GM made three “types” of 12-bolt carriers (Type 2, Type 3, and Type 4). Each was designed for use with a limited range of ring-and-pinion gears. Changing gears between carrier types is possible, but will require the use of gear shims or thicker-than-stock ring gears. The Type-2 differential is typically found in cars that came with smaller-displacement V8 engines, and were not intended to be used in high-performance applications.
Because of the thickness of the flange where the ring gear mounts, gear ratios numerically higher than 2.73 cannot be used on a Type-2 carrier. A numerically larger ring gear will physically bolt to the differential, but you will not be able to get the pinion-and-ring gear to properly mesh.
Type-3 carriers have a thicker ring gear-mounting location and are designed for use with original gear ratios ranging from 3.08 to 3.73. Finally, Type-4 differentials have the thickest ring gear-mounting flange and carry gear ratios ranging from 3.90 to 6.14.
Type-2 differentials are probably the most plentiful since they are found primarily in non-performance Chevrolet passenger cars. These carriers were not intended for any serious performance applications, but are suitable for a weekend cruiser that will not be using sticky tires and traction enhancers.
All OE 12-bolt Positraction rearends use 30-spline axles. The 12-bolt’s axles are held in place with C-clips, and these axle retainers are considered to be a downside to the 12-bolt. If an axle breaks, there is nothing holding the wheel and tire into the rearend. Many car owners have dropped the clutch at a red light or the drag strip starting line, only to watch their car’s rear wheel roll past them. The 12-bolt can be upgraded by installing C-clip eliminators. To do this, the axles need to be removed and modified to install the C-clip eliminators. If you are planning to – even occasionally – launch your car with slicks and/or traction-enhancing suspension parts, it’s a good idea to have them added.
Hopefully, this short outline about the venerable 12-bolt will give you some insight when you head out to the next swap meet. What’s more, you can print this article and take it with you to help you decipher what you are looking at. That way, you will have an idea of whether that diamond in the rough before you is what you need or not.