Chevy 12 Bolt Identificationby Moss Strohem
The GM 12-bolt rear axle assembly receives its name due to having 12 bolts on the rear cover of the housing. This rear axle was available in GM's high-performance intermediate cars, full-size vehicles and light-duty trucks.
The 12-bolt rear end was introduced in the early 1960s and was used from 1964 to 1972 in GM cars. It was used in truck applications until 1982.
Its larger size and stronger design make it a popular upgrade for improved performance as the 12 bolt is able to handle large amounts of power.
Externally, the 12-bolt rear end is identifiable by the cover, but internally (once the cover is removed) it can be identified by the size of the ring gear, which is 8-7/8 inch in diameter.
12-bolt axles can be identified by casting numbers on the bottom of the cast-iron center-section and on the front of the passenger side axle tube. This identifies the car make and model, gear ratio, date of casting and assembly and factory of origin.
The short production run (eight years for cars) and classic car restoration era has driven values of this rear axle up considerably.