GM 10-bolt Differential
Technical Information

Basic Information
Historic Changes
GM 10-bolt Identification
Carrier Breaks
Casting Numbers

Rearend Quick ID - An 8.2 Pontiac 10-bolt has 2 scallops (cutouts) on each side of the rear cover, whereas an 8.2 Chevy-style has a round cover with an indentation running across the cover (like the 12-bolt Chevy) and 2 casting projections on the top of the rear. The Pontiac 8.2 inch has its axles retained at the brake end of the housing and all the other rears have C-clips retaining their axles in the housing (in stock form). The 8.5 GM corporate rear has a rounded cover with two casting projections that face downward. An 8.5 posi uses plate clutches like a 12-bolt, whereas the 8.2 Chevy and Pontiac both use cone-type clutches in the posi unit.

NOTES

• All 8.5" 10-bolt carriers and ring/pinions will interchange, with the exception of the 30-spline truck units.
• From 1964-67, the housing dimensions measure 56.5 inches, from flange to flange. Total measurements with the drums in place is 60.5 inches. 1968-72 rearend housings measure 58.5 inches from flange to flange, and with the brake drums in place, the total dimensions measure 62.5 inches.
• Internal components have a characteristic that is common to the housing used: 10-bolt pieces fit other 8.125" 10-bolt housings (the rearend used in Chevrolets), and 12-bolt pieces interchange with other 12-bolt housings (not the one from the Chevrolet pickup or Oldsmobile, which has a 12-bolt cover and a 10-bolt gear, which measures 8.3"). This means that a posi carrier will retrofit in place of a standard differential.
• Axle shafts are common to the housing used, and due to the usual characteristics like overall length and spline count, the shafts only interchange with the housing that is used. (e.g. 12-bolt shafts fit other 12-bolts, and 10-bolt shafts fit other 10-bolts.)
• Rear spring mounting pads: 1964-66 rearends used a flat pad with a hole drilled in the center. 1968-72 rearends have circular spring mounting pads, which are 3/4" higher that the early flat pad. 1967 was a transition year, in which A-cars might have a 1964-66 style rearend, a 1968-72 rearend (which is wider, and commonly available), or a "hybrid" rearend, which will have the 58.5" width, but with the early spring mounting pad and trailing arm brackets.
• Rear upper control bushing eyes differ, and the positioning of the bushing eyes will differ. 1968-72 rearend housings will have a 3/8" forward positioning, which is farther than 1964-67 rearends.
• 1964 was the only year that the rearend bushings are small, and any upper trailing arm (from any GM division) will fit 1964 rearend housings.
• If bolting a Chevy 12-bolt rearend into your Cutlass, you will notice the U-joint at the input yoke is a different diameter. There is a U-joint that will adapt a BOP driveshaft to the 12-bolt Chevy rear.
• 10-bolt and 12-bolt drum brake assemblies will interchange.

REAR TRAILING ARMS

• The rear trailing arms (or control arms) consist of four arms that connect the rearend to the frame, and the setup consists of two long and two short arms. The lower trailing arms for use with a sway bar is unique, and the usual characteristics is that the arm is boxed and gussetted. The upper arms vary, and there are two part numbers in the Chevrolet Parts Interchange Manual that separate 1964-67 and 1968-72 Chevelles and BOP A-Bodies.The lower arms are interchangeable (all years), and a car that did not have a sway bar can be modified to fit.

• Upper trailing arms of 1964-67 vintage interchange, and they are 1" shorter, which will not fit into 1968-72 A-cars. 1968-72 A-cars have longer upper arms, and when switching upper arms, be careful here, in which the pinion nose angle might be affected. According to Inside '64-'72 A-Bodies, there are 10 different rear upper control arms offered. Other characteristics include clearance bulges, common with 12-bolt differentials in A-cars, and adjustable upper arms, optioned on Oldsmobile A-bodies (F-85, Cutlass). 442s had boxed upper arms, and this is a sought-after item in a restoration.

• On many high-performance and 4-speed-equipped 1968-72 A-cars, there is a triangulation brace bracket that is standard. This stiffens the chassis, and tubular versions are available from Edelbrock and Hotchkis Performance.

• The left and right trailing arms are interchangeable.

HISTORIC CHANGES

11 1/2" Irregular Cover - 10 Holes
8.20" Diameter Ring Gear
1.438" Diameter Pinion - 27 Splines "B" or "P" Axle Only
Non C-clip type

64-70 Olds F-85
64-71 Tempest/GTO
67-71Firebird

These units were used for a number of years and are strong, but there is a certain lack of parts availability from the factory.

Top
11" Oval Cover - 10 Holes

Bottom
10 5/8" Irregular Cover - 10 Holes

8.20" Diameter Ring Gear
1.438" Diameter Pinion - 25 Splines

65-70 Chevrolet
64-72 Chevelle
64-70 Chevy II
67-70 Camaro
70-72 Nova

Most Chevy-style 8.2" 10-bolts use the 11" cover with an inverted "V" groove (for internal lubrication) across it's face, just like the 12-bolt version. Some have a 10 5/8" irregular cover. All have slip-in axles retained by C-clips. The pinion nut will be 1 1/8".

Top:
11" Oval Cover - 10 Holes

Bottom:
10 5/8" Irregular Cover - 10 Holes

8.50" Diameter Ring Gear
1.626" Diameter Pinion - 30 Splines
Plate clutches

70-76 Chevrolet
70-76 Chevelle
70-75 Chevy II
70-81 Camaro
70-81 GTO/Firebird
70-76 Olds F-85
72-81 Z-28

Most 8.5" 10-bolts have a bulged-out 11" cover (for ring gear clearance) with big square lugs on the housing below the cover (red arrows). A few use the 10 5/8" irregular cover. The pinion nut will be 1 1/4". All corporate 8.5" 10-bolts have a 30-spline pinion, but various years & styles had C-clips or bolt-in axles.

10 3/8" x 11" Irregular Cover -12 Holes
10-Bolt 8.5" Diameter Ring Gear - 27 Splines
1.626" Diameter Pinion
"O" Axle Only 12 Bolt Cover - 10 Bolt Gear
Plate clutches

68-70 Cutlass and 442

This is the Olds version of the the Chevy 12-bolt rearend. The bearings in this rearend are smaller than the Chevy version as is the ring gear diameter, which is 8½ vs. 8 1/8 for the Chevrolet unit. It is identified by its almost round rear cover. It comes with bolt-in axles.

8 5/16" x 10 9/16"  Oval Cover 10 Holes
10 Bolt 7.5" & 7.625" Diameter Ring Gear
1.438" Diameter Pinion - 27 Splines

77-98 All GM Models
82-98 Camaro, Firebird Trans-Am Z28
83-98 S10, S15 Trucks (Rear)
85-98 Safari, Astrovan
77-87 El Camino
Will not fit Monza, etc.

Second-design 7.5"/7.625" 10-bolt housings are similar in appearance to the 8.5" 10-bolt housing. The 7.5"/7.625" uses an 8 5/16 x 10 9/16 oval cover with small pointed lugs on the housing below the cover (red arrows).

9" Ford bolt-in housing made to fit 1964-1972 GM A-body cars - $359 from Moser Engineering

10 Bolt 8.5" & 8.2" Performance Cover - $160 from Moser Engineering

GM rearend carrier breaks

• 7.5" - 3.08 & down / 3.23 & up
• 7.75" - 2.77 & down / 3.27 & up
• 8.2" - Chevy 2.76 & down / 3.07 & up
• 8.2" BOP  - 2.76 & down / 2.94 to 3.23 / 3.31 & up
• 8.5" - 2.56 & down / 2.76 & up
• 12-Bolt Passenger - 2.76 & down / 3.07 to 3.90 / 4.10 & up

GM Carrier Casting Numbers

Sources

• "Inside '64 -- '72 A-Bodies", Car Craft, March 1998
• "Beefing Up GM 10-Bolts", Car Craft, August 1984
• Chevelle Online Interchange Manual
• Scott's Nova Resource Site

OLDSmobility.com Related Links

• 1967 Oldsmobile Differential Identification and Ratios
• Rebuilding The GM 10-bolt Posi
• Nineteen Startling Chevy 12-Bolt Differential Tips