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  Chevy, a division of General Motors, introduced the 12-bolt rear axle assembly in 1965. Another common name for this assembly is the 12-bolt truck differential. The design gave the axle assembly the strength and durability to handle the big engines in Chevy cars and trucks. Chevy 12-bolt rear axle assemblies were found on most Chevy and GM cars or light trucks. The assembly was mass-produced until 1972.
  
  

Pinion

• A pinion is a gear with small teeth that mesh with a larger wheel or rack. The pinion of the 12-bolt is 1 1/2 feet below the centerline of the ring gear. The result is less loss of horsepower and lower temperatures in the gear lubricant. The pinion gear shaft is shimmed to get proper pinion depth. It is 1.625 inches in diameter.
  
  

Ring Gear

• A ring gear is a circular gear attached to the differential case. The ring gear carrier for the Chevy 12-bolt is 8.875 inches. Two internal caps hold the carrier in place.
  
  

Differentials

• Differentials distribute power to each axle as needed depending on the load. They allow the wheels to spin independently. The differentials for passenger cars with the Chevy 12-bolt assembly are type 2 series. The ratio for this series is 2.29:1 to 2.73:1. High performance cars have type 3 differentials, which is a ratio of 3.8:1 to 3.73:1. The type 4 series ratio is 3.9:1 up to 6.14:1. These ratios represent the ring and pinions accepted by the differential.
  
  

Yokes

• When properly assembled, the pinion splines into a yoke that turn the gear. Yokes combine with the rear universal joint of the driveshaft to enable the axle to function properly. The 12-bolt uses the smallest style universal joint -- the 1310.
  
  

Lubricant

• High quality oil should be used in the Chevy 12-bolt. The level and housing should be filled along with the rear axle assembly. It is advised that only two quarts of oil plus any additive be used because it is easy to overfill.