Known alternatively as the GM 8.875-inch (referring to its ring gear diameter), Chevrolet's 12-bolt rear end is one of the most celebrated in the American hot-rodding scene. Lighter and more efficient than its Ford 9-inch and Dana 60 rivals, the 12-bolt enjoys as much use amongst brand-loyal off-roaders as it does among drag and road racers.
The 12-bolt derives its name from the number of bolts in its differential cover, which distinguish it from all other General Motors (GM) axles. The 12-bolt can use either an oval-shaped or irregularly shaped gasket, but cover bolt torque comes in at 20 foot-lbs. regardless of cover type.
Pinion bearing pre-load, a critical parameter, determines the amount of resistance to movement imposed by the pinion bearing. Miss this crucial measurement by even a few percent and your bearings will wear out far before their time, and the differential case itself may crack in half under high temperatures and extreme stresses. Used pinion bearings should resist rotation at the rate of 10 to 15 inch-lbs. (not foot-lbs.) of torque; new bearings are a little tighter and should check in at 24 to 24 inch-lbs. of pre-load.
Carrier backlash, the distance between the differential carrier unit and the carrier bearing housing in the case, largely determines the contact area between the ring and pinion gears. The 12-bolt uses shims set between the carrier and carrier bearings to set backlash, which should ideally land between 0.005 and 0.009 inches. However, backlash may vary depending on the desired contact patch between ring and pinion gear.
The ring gear bolts to the differential carrier and transmits power from the pinion to the differential unit itself. Torque the ring gear bolts to 90 foot-lbs., use medium (blue) thread locker to hold them in place.
The differential's side bearing caps, equivalent to the main caps in your engine, hold the carrier unit to the case. Always use new carrier bearings, plenty of assembly lube and blue thread locker when installing the side bearing caps, and torque the retaining bolts to 60 foot-lbs.
