GM Axle ID and Info

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10-Bolt

image is slighty irregular
Donor Vehicles:K10-20, FRONT 77-87, BLAZER FRONT 77-91, C/K10 REAR 77-C, BLAZER REAR 80-94, B CAR W/ ABS (OEM GM R&P) 90-97
Applications: Front and rear
Type: Semi-floating
Spline count: 28 and 30
Factory ratios: 2.56:1 through 4.56:1
Maximum tire size for stock axle: 35-inch
Strong point: Strong ring-and-pinion for its size, large ring-gear bolts, spider gears and pinion diameter (when compared to its competition, the Dana 44)
Weak point: Avoid the centrifugal-force-triggered Gov-Lok locker
Junkyard jewel: Find a 10-bolt-equipped Chevy Blazer or Suburban built after November 1989 through 1991 because it will have 30-spline axleshafts
Building secrets: The axletubes can spin on the centersection, so notch the housing where the axletube meets the centersection and re-weld
Aftermarket alternatives: Custom Differentials.
Notes: Unfairly, this axle has been the redheaded stepchild in the family of GM axles. Why is this unfair? Because it boasts an inner pinion bearing that is stronger than the one found in a 12-bolt axle, and the pinion-shaft diameter is also larger. Before you run out and snag a 10-bolt, though, be aware that GM offers seven different variations of this axle.

GM 12-Bolt

image is slightly irregular
Donor Vehicles:C10/C15 64-82, G10/G15 64-81, G20/G25 69-81, K10/K15 64-82
Application: Rear
Type: Semi-floating
Spline count: 30 (after '68.)
Factory ratios: 2.50:1 through 4.56:1
Maximum recommended tire size for stock axle: 35-inch
Strong point: Larger-diameter ring gear than 10-bolt (8.875-inch)
Weak point: The pinion bearings are small and fail often
Junkyard jewel: Some '76-or-older Chevy ½-ton trucks had a good Eaton coil-spring-type limited-slip
Building secrets: Some ratios allow the use of a 12-bolt automobile gearset, which, interestingly, uses a larger-diameter pinion shaft
Aftermarket alternatives: Currie Enterprises, Custom Differentials, DTS Custom Service
Notes: This axle was manufactured in both 5- or 6-lug bolt patterns. If you trash the 12-bolt in your '73-to-early-'80 4x4 Chevy truck, you'll be happy to know that you can simply install your 6-lug axleshafts into a two-wheel-drive 12-bolt axle and return to the trail.

14-bolt

Donor Vehicles:P/C/K 1 TON 73-99
Application: Rear
Type: Semi-floating
Spline count: 30, 33
Factory ratios: 3.23:1 through 5.14:1
Maximum tire size for stock axle with 10½-inch ring gear: 44-inch
Strong point: It features a removable pinion support and a pilot bearing at the end of the pinion shaft for added strength
Weak point: The left-side carrier cap is known to break through the bolt hole
Junkyard jewel: Find one on a '73-to-current ¾- or 1-ton Chevy truck. The newer the axle, the better
Building secrets: Replace the Gov-Lok diff with an aftermarket unit
Aftermarket alternatives: Custom Differentials.
Notes: The GM 14-bolt came in both a 9½-inch ring gear and a 10½-inch ring gear, and they're totally different animals. The latter is the more desirable, and its only resemblance to the 9½-inch is the number of cover bolts.
Pic of the "other" 14 bolt: