GM 9-1/2-INCH & 10-1/2-INCH 14-BOLT
GM produced two 14-bolt axles through the years: the 9-1/2-inch and the 10-1/2-inch (shown). The 10-1/2-inch is the stronger of the two. Both use 14-bolt covers, but the 9-1/2 is more egg-shaped (like a larger GM 7-1/2), while the 10-1/2 has straight sides and sharper corners. The 9-1/2 is an integral pinion design where the pinion is removed through the rear cover plate. The 10-1/2 is a removable pinion design, which allows the pinion to be removed from the front of the housing without pulling the rear cover. Other distinguishing features of the 10-1/2 are the 6-bolt removable pinion support and full-floating axles.
Applications: '88-'96 C10/C15/C20 (9-1/2-inch), '88-'96 K10/K15, '81-'96 K20, '73-'92 K2500/K3500 (10-1/2-inch)
Ring-gear diameter: 9-1/2 in, 10-1/2 in
Pinion-gear diameter/splines: 1.876 in/30 splines (9-1/2), 1.750 in/30 splines (10-1/2)
Available gearsets:3.42, 3.73, 4.10, 4.56, 4.88, 5.13 (10-1/2 only)

GM 12-Bolt
Two versions of the GM 12-bolt exist but have nothing more than the bolt count in common. One is a car version with a grooved cover plate, while the truck-version cover plate is raised. The 12-bolt is integral and semifloating, but isn't a big step up from the 8-1/2-inch 10-bolt because the axleshafts and housings are very similar. A drawback to both the 10- and 12-bolt axles is that the axleshafts are retained by C-clips, which, if broken, can be a serious hazard.
Applications: '64-'82 K10/K20 and Blazer
Ring-gear diameter: 8.875 in
Pinion-gear diameter/splines: 1.438/30 splines
Available gearsets: 3.07, 3.42, 3.73, 4.10, 4.56, 4.88, 5.13, and 5.38