How do you identify a GM 10-bolt?

The most common GM 10-bolt differentials are the GM 8.5" and GM 8.6", both of which can be identified by 10 bolts on the rear differential cover and an almost perfectly round differential cover. To further identify the GM 10-bolt, check the style of axle and spline count.

From 1988 through current production as of 2015, GM 10-bolt differentials are c-clip style, 30-spline axles. Prior to 1988 the axles had 28-splines. The GM 10-bolt ring gear measures 8 1/2 inches or 8 5/8 inches. Replacement parts for the GM 8.5" and GM 8.6" 10-bolt differentials are interchangeable, except for the bearings for the differential carrier.