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.
Is It A Posi?
Everyone's heard the tip about checking your vehicle's rearend to check for a "posi-traction" or "limited slip" unit as opposed to an "open" unit....and that is to jack both wheels off the ground and try to spin one tire. If the opposite tire spins in the same direction, it's a posi. If it spins in the opposite direction, it's not. However, this is NOT an entirely accurate way of correctly determining the existance of the posi.
A posi that is set up right will cause both wheels to turn the same direction with the trans in N. When the trans is in P you should not be able to turn the wheels. The problem is that when the clutches wear, the unit loosens up and begins to act like a "open" or standard unit. In this case the wheels will turn in oposite directions with the trans in P. The only way to be sure is to remove the cover and look at it. If it has the preload springs, plates and clutches, it's a posi. If it is a posi and it is acting like a standard unit, you need to have the clutches replaced and the preload reset.
The only way to know for sure it to take the cover off the rear end and look at the carrier! Look for the two plates with four springs between them, or the "s" spring in the later 10-bolts. Even if both wheels were to spin the same direction in the above scenario, it could mean that the rearend has welded itself together from too many one-legged burnouts...or it might even have a racing-only spool installed. NOT the ticket for a street-driven car.
According to the 1970 service manual, for a new posi-traction differential, the torque required to rotate one wheel while the other wheel remains stationary should be 70 ft. lbs. minimum. For a used posi differential it should be no less than 40 ft. lbs.
TO CHECK THE RING AND PINION GEAR RATIO: Remove the inspection cover and check the side of the ring gear for two numbers...for example, 43:13. This means that there are 43 teeth on the ring, and 13 teeth on the pinion. 43/13 = 3.31 ratio. BotXXXXX XXXXXne: If you're looking to purchase a rearend from a third party, never take the seller's word for the gear ratio. Take a moment and do the inspection of the gears personally.
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.
Evolutionary Time Period
Years / Models Used In
Ring Gear Dia.
Factory Gear Ratios (Separated by Case Sizing)
Positraction Case Casting No.
No. of Splines
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.
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".
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
GM Carrier Casting Numbers