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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.
Bottom line: 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. |
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 |
|
GM 8.5
"O" Axle
8.50" Diameter Ring Gear
8 5/16" x 10 9/16"
Oval Cover 10 Holes
