['ol Pinion head]

Originally Posted by scott70

OPH...you spoke of the 12 bolts that came in the 455 A-bodies. My 70 455 gto still has its original 3.31 posi 12 bolt. What are the differences between that and the chevy 12 bolt? What parts will interchange? Thanks

On the 12 bolt c-clip axles that found their way under the '70-72 455 GTO's & '71-72 455 LeMans, they were all built of the McKinnon plant in Canada. There was also some usage of this style rear in '68 & 69 Olds A-body's & '69-70 Buick Skylarks. These specific 12 bolts had different axles, backing plates, different drums, and a 3R pinion yoke. All different than the common Chevrolet application 12 bolt rear.

The McKinnon cast 12 bolt housing will also not have a date code cast into the lower pass side webbing. Instead, the housing is date coded on the front of the lower "lip" area of the housing, visible from the front of the housing. On many McKinnon 12 bolts this date casting may not even be legible, the casting date is quite small.

McKinnon (GM of Canada division) also built some 12 bolt "Chevelle" rears for Chevy usage, I've ran across several out of '69 & '70 "Chevelles". These will have the Chevy specific ID codes, as well as the standard duty Chevelle ('70 F-body) length axles, smoothie style drums, & the very common style "Chevy" backing plates with the spotwelded flange.

[Schurkey]

Originally Posted by 'ol Pinion head

The 8.5 rear design was introduced for 71 model GM passenger cars. GM 1/2 ton pickups would have to wait til '77 model year...

...B body:
for '71, most GM divisions used a c-clip axle 8.5 B series rear in their base B series models.

Originally Posted by Cliff R

You mentioned someplace about 1977 half ton trucks getting 8.5's by 1977.

Well. Aren't I embarassed.

I have learned something new, and I wasn't expecting this.

I was sure that the 8.5" wasn't used on B-bodies until the major downsizing in '77, when the B-body was shortened and lightened; basically a boxy body on a modified A-body frame.

And I thought the first use of the 8.5 on pickups was for the '88 model year which started earlier than usual in '87. The 8.5 was upgraded in late '88 for '89 model year with 30-spline axles and side-gears, maybe bigger bearings.

I believed that the 8.5 was a wonderful axle for compact and intermediate cars using small-block engines, but had no business behind a big-block or under a full-size car; and the 8.5 is a hot mess under a full-size pickup. Seen too many 8.5s shattered and exploded under GMT400 trucks. Strangely, I've never seen a blown-up 8.5 under a Squarebody truck.

['ol Pinion head]

Abused Eaton Govenor locks have taken out more gearsets & 8.5 rears in pickup housings than anything else. Very rarely have I ran across a blown up 8.5 ring & pinion even in a street strip car build.

What i ran into alot years ago was many young owners thought with a lift & big tires, their "truck" was ready to go to the river. In the course of playing at the river, doing big donuts, etc, the Gov-lock rear was the weak spot. The Gov-locks just do not like two big rear tires/rear of the truck being thrown in an arcing manner.

When a Gov-locks blows & takes out the ring & pinion, they typ take out the drivers side cap, as well as many times taking out the cap mating area of the housing. From there, the owners typical action was a search for another rearend. The chance of finding just a core rear or housing in the cheap yards was usually very slim as the 4WD 8.5 housings & axles are of a different length than the 2WD variety. For a while, I pulled that particular 4WD pickup housing in a few Pick-N-Pulls, eventually rebuilding them with a new clutch type Eaton carrier, or in a few cases with used 30 spline Auburn posi carriers out of B series 8.5's (impallas & Roadmasters).

[scott70]

Originally Posted by 'ol Pinion head

On the 12 bolt c-clip axles that found their way under the '70-72 455 GTO's & '71-72 455 LeMans, they were all built of the McKinnon plant in Canada. There was also some usage of this style rear in '68 & 69 Olds A-body's & '69-70 Buick Skylarks. These specific 12 bolts had different axles, backing plates, different drums, and a 3R pinion yoke. All different than the common Chevrolet application 12 bolt rear.

The McKinnon cast 12 bolt housing will also not have a date code cast into the lower pass side webbing. Instead, the housing is date coded on the front of the lower "lip" area of the housing, visible from the front of the housing. On many McKinnon 12 bolts this date casting may not even be legible, the casting date is quite small.

McKinnon (GM of Canada division) also built some 12 bolt "Chevelle" rears for Chevy usage, I've ran across several out of '69 & '70 "Chevelles". These will have the Chevy specific ID codes, as well as the standard duty Chevelle ('70 F-body) length axles, smoothie style drums, & the very common style "Chevy" backing plates with the spotwelded flange.

Is the carrier the same as the chevy 12 bolt? Didnt know if a chevy 3 series posi carrier would fit if anything went wrong with mine.My diff cover looks identical to a chevy 12 bolt cover. Also the rear pinion yoke takes a U bolt and not a strap for the U joint.

['ol Pinion head]

the carriers & ring & pinions are the same.

[alsalp]

what are the carrier breaks for bop 8.2's?

[b-man]

Originally Posted by alsalp

what are the carrier breaks for bop 8.2's?

2.56 and 2.78

2.93, 3.08 and 3.23

3.36 and numerically higher.

[Cliff R]

"Cliff, On the half tons, the base 1/2 tons began receiving the 8.5 10 bolts in '77, while the Big 10 Chevys & Heavy Half GMC's received the 12 bolt truck axle."

I've seen the 12 bolts as late as 1979 so knew they still offered them in some applications. My 1979 K-5 Blazer had a 12 bolt in it, and it was the only 12 bolt I've ever seen with the Eaton Locking carrier in it.. Those do get a bad rap for blowing up, but I've never had the first issue with one and have owned many GM trucks, Blazers and Suburbans with them. The death sentence for them is putting one wheel on wet grass and the other on pavement and whacking the piss out it. Over zealous youngsters also try to do hard right hard turn burnouts with them at every stop sign they come to and they don't last very long doing that either.

I've found them difficult to rebuild and service, and had to make special tools to press the governor and latch assembly out of them. I still consider them an excellent part for what they were designed for, but a very poor choice for drag racing diffs.......

https://www.youtube.com/watch?v=ftyJvIO0DZ8

[NeighborsComplaint]

[QUOTE=So tell me... did Pontiac ever put an 8.5" in one of our beloved (Pontiac) A-bodies? [/QUOTE]

Short answer ... Nope. BOP 8.2" 10 bolt w. bearing flange axle retainers was specific to Buick, Olds and Pontiac. Chevy had its own 8.2" 10 bolt which utilized C clips for axle retention before the corporate 8.5" 10 bolt became the norm.

Good article here: https://www.hemmings.com/stories/art...o-p-rear-axles

['ol Pinion head]

Originally Posted by NeighborsComplaint

Short answer ... Nope. BOP 8.2" 10 bolt w. bearing flange axle retainers was specific to Buick, Olds and Pontiac. Chevy had its own 8.2" 10 bolt which utilized C clips for axle retention before the corporate 8.5" 10 bolt became the norm.

Good article here: https://www.hemmings.com/stories/art...o-p-rear-axles

Quite a few errors in that "good" article.

Pontiac most certainly installed 8.5 rears in nearly every '73 A-body.
Mention "nearly every" as there were the 8 7/8 MP series rears that came in the SW's & for a few years in numerous 455 powered GrandAms, etc,