[fireprix]

I have had this question for years and was curious to see if anyone knows or has opinions on this.

Most Pontiac fans know that Pontiac’s Ram Air V heads were based off the Ford Tunnelport heads. Ford as far as I know never said much if anything about this.

What I want to know is this: Pontiac was the division of G.M. that built the odd 8 7/8” rear that had 31 spline axles (later 28), a 12 bolt carrier and a 10 bolt cover, often referred to as the 8 3/4 to prevent confusion with the muscle car 12 bolt as far as parts. This is the rear that was used mostly in full sized G.M. cars and station wagons from about 1965-1982 in somewhat different forms. The posi units were built by Eaton who no longer supports it. It also looks strikingly like the Ford 8.8” rear. I actually have one of these in my 75 GP and was able to track down a good used posi and set of gears for it from a reputable source who sent a kit of what was needed to install in my housing. Some of the parts in the kit had Ford part numbers. Did Pontiac give Ford the rights to this in return for borrowing the tunnelport design from Ford years back? The Ford 8.8 came out shortly after Pontiac discontinued production of their rear.

Also perhaps someone could look into the possibility of if there are any parts interchange between the G.M. and Ford rears. Occasionally questions come upon the forums about this rear and most are discouraged from using them due to the zero aftermarket support. It is a shame because these rears are very strong and in some ways stronger than the muscle car 12 bolts. Unfortunately they weren’t made to fit many performance cars but could be a budget bulletproof rear. Any thoughts?

[Half-Inch Stud]

Reverse that. The PMD head Developer left PMD after the R&D got cut, and went to Ford.

[jerry455]

Pontiac also used Ford transmissions for a while in the 60's. The Dearborn trans was sourced from Ford when Pontiac could not get the GM trans for whatever.

[Steve C.]

This from one web site.....

Things got weird in 1963. Not only did Ford and Pontiac simultaneously share the Super Duty nomenclature, Pontiac sourced the Ford hood scoop for factory installation on the 1963 421 Tempest A/FX (13 built) and 421 "Swiss cheese" B/FX Catalina Super Duty (14 built) drag-race package cars. The Ford part number was even provided to the NHRA for verification by Pontiac!


.

[Simple Man]

Blame Bunkie!

I thought Olds was the 12 bolt cover, on a 10 bolt gear set.

[F ROCK]

Originally Posted by Simple Man

Blame Bunkie!

I thought Olds was the 12 bolt cover, on a 10 bolt gear set.

Yes?
I had one in my lemans for many years.
We referred to it as the "type O" rear, as we'd heard it was an oldsmobile rear.
smooth 12 bolt cover w/ 10 bolt ring gear.
It held up well.
I eventually sold it.

[Simple Man]

Originally Posted by F ROCK

Yes?
I had one in my lemans for many years.
We referred to it as the "type O" rear, as we'd heard it was an oldsmobile rear.
smooth 12 bolt cover w/ 10 bolt ring gear.
It held up well.
I eventually sold it.

I had a similar rear axle in my 65 Tempest. It was from a 69 Cutlass, with 3:55 gears. Held up well behind a 400 with a T350.

['ol Pinion head]

'67-70 Type "O" Olds rears were manufactured by Olds for it's divisions A-body's & B-body's. Only Olds cars originally came with them, though many Type "O" Olds rears have over the decades found their way under other GM A-body's. In strongest axle form, for 2 years, the A-body version was avail in 3.42 & 3.91 Anti-Spin ratios. Out in the hsg racks, have a 3.91 Anti-Spin core rear out of '68 442. While the 31 spline axle housing versions have beefy axles, the Type "O" rears lack the pinion support of its replacement, the 8.5 A-body rear.

['ol Pinion head]

The use of 28 spline & 31 spline axles was wider usage than just Ford 9" rears & the Pontiac's 8 7/8 MP series rear. Carrier bearing sizing followed the size of the housing end of the carrier. There was no combined development between Ford & PMD on their rearend designs. Each GM Automotive division in '70 models & earlier had a LOT of autonomy in the design & production of their B series rears. GM then phased over to a corporate approach for its most common rear axle designs. Buick was able to continue on with a version of its 9 3/8 rear in Electras & Centurions & under its Riv's up through '74 models; very hard to find w 3.23 or 3.42 & a posi carrier. 8.5 c-clip axle rears went in base horse entry level '71 B series; Impallas, LeSabres, Catalina's, etc. For '65-mid 70's, Eaton & Warner-Motive supplied a good chunk of posi units for all of GM's Salisbury style automobile rears.

GM automotive axle housing design, pre '71, as well as axle design, including sizing of axle bearing, & spline style & count were specified by each GM Division's Engineers. Pontiac Engineering developed the "P" series 8 7/8" rear for the '65 model Pontiac B series. it's mfg, from all I've examined, was only in the Pontiac MI axle plant. The '71-76 8 7/8" M(odified) P series rear has its roots in its predecessor, the 8 7/8 "P" series. The latter, 8 7/8 MP series made it under many '71-76 B-O-P division B series.

Eventually, the 8 7/8 MP series B series rear received 28 spline axles. '77-83 was the last rendition of this style rear, ring & pinion sets from the last version will not fit in earlier housings, the pinion is longer. In the A-body chassis, '73-77 A-body wagons received an 8 7/8" MP series rear. There was also the '73-75(?) Pontiac A-body 8 7/8 MP series conventional 4 link style rearend, which at least for a while came under 455 powered GrandAm's, Luxury LeMans, etc. Have only had a few of this style housing, though have pulled parts for many. All the 8 7/8 MP series posi carriers I've ever pulled were mfg by Eaton. Fairly easily rebuildable with new GM clutches, though side gears & spiders had to be sourced used. Eventually in some literature, this style rear is noted as an 8 3/4. The latest version was last used in certain early 80's Cadillacs.

[unruhjonny]

Originally Posted by jerry455

Pontiac also used Ford transmissions for a while in the 60's. The Dearborn trans was sourced from Ford when Pontiac could not get the GM trans for whatever.

I have heard a version of this from a couple different sources, and I think it's incorrect.

Contrary to this scenario, I have understood that GM (in this case Pontiac) sourced the Dearborn three speed not because of any shortage, but because it was known that the Saginaw three speed was incapable of surviving behind higher torque applications;
in the case of Chevrolet, I had understood that any such engines required either an optional heavy duty automatic, or a Muncie four speed to get around this problem.
I had come to understand also that this was precisely the reason why the 'Muncie 330' was developed;
It was developed as an improvement on the Saginaw three speed, to survive behind high torque engines, and allowed Chevrolet to have a heavy duty three speed without a "FoMoCo" emblem on it.

[OG68]

Even though Chevrolet/GMC trucks used the Dearborn as its standard three speed through the early eighties

[fireprix]

Originally Posted by 'ol Pinion head

The use of 28 spline & 31 spline axles was wider usage than just Ford 9" rears & the Pontiac's 8 7/8 MP series rear. Carrier bearing sizing followed the size of the housing end of the carrier. There was no combined development between Ford & PMD on their rearend designs. Each GM Automotive division in '70 models & earlier had a LOT of autonomy in the design & production of their B series rears. GM then phased over to a corporate approach for its most common rear axle designs. Buick was able to continue on with a version of its 9 3/8 rear in Electras & Centurions & under its Riv's up through '74 models; very hard to find w 3.23 or 3.42 & a posi carrier. 8.5 c-clip axle rears went in base horse entry level '71 B series; Impallas, LeSabres, Catalina's, etc. For '65-mid 70's, Eaton & Warner-Motive supplied a good chunk of posi units for all of GM's Salisbury style automobile rears.

GM automotive axle housing design, pre '71, as well as axle design, including sizing of axle bearing, & spline style & count were specified by each GM Division's Engineers. Pontiac Engineering developed the "P" series 8 7/8" rear for the '65 model Pontiac B series. it's mfg, from all I've examined, was only in the Pontiac MI axle plant. The '71-76 8 7/8" M(odified) P series rear has its roots in its predecessor, the 8 7/8 "P" series. The latter, 8 7/8 MP series made it under many '71-76 B-O-P division B series.

Eventually, the 8 7/8 MP series B series rear received 28 spline axles. '77-83 was the last rendition of this style rear, ring & pinion sets from the last version will not fit in earlier housings, the pinion is longer. In the A-body chassis, '73-77 A-body wagons received an 8 7/8" MP series rear. There was also the '73-75(?) Pontiac A-body 8 7/8 MP series conventional 4 link style rearend, which at least for a while came under 455 powered GrandAm's, Luxury LeMans, etc. Have only had a few of this style housing, though have pulled parts for many. All the 8 7/8 MP series posi carriers I've ever pulled were mfg by Eaton. Fairly easily rebuildable with new GM clutches, though side gears & spiders had to be sourced used. Eventually in some literature, this style rear is noted as an 8 3/4. The latest version was last used in certain early 80's Cadillacs.

Lots of good info here. I wasn’t implying Ford and Pontiac worked together to develop this rear. What I was trying to find out was:

1) Did Pontiac possibly give Ford this rearend design in return for Pontiac’s borrowing the Ford design of their tunnelport heads for their Ram Air V heads. It seems to me that the Pontiac 8 7/8 MP rearend is very similar to Ford’s 8.8 rearend. Also shortly after production of the 8 7/8 MP rear from Pontiac ceased Ford came out with the very similar 8.8.

2) Is it possible (since the install kit I got had parts that were clearly Ford parts) that some of the parts may interchange between the 2? They appear very similar. The Ford 8.8 has tremendous aftermarket support while the Pontiac 8 7/8 MP has none. In fact Eaton lists nothing regarding the 8 7/8 MP.

[unruhjonny]

Originally Posted by OG68

Even though Chevrolet/GMC trucks used the Dearborn as its standard three speed through the early eighties

1969 through to at least 1974 the three speed was a Muncie 330;
I do not know what GM decided to do after they stopped using this three speed.
It is entirely possible that GM did an about face and went to another supplier rather than manufacture them in-house, as this is precisely what they did with the four speeds found in cars.

['ol Pinion head]

Originally Posted by fireprix

Lots of good info here. I wasn’t implying Ford and Pontiac worked together to develop this rear. What I was trying to find out was:

1) Did Pontiac possibly give Ford this rearend design in return for Pontiac’s borrowing the Ford design of their tunnelport heads for their Ram Air V heads. It seems to me that the Pontiac 8 7/8 MP rearend is very similar to Ford’s 8.8 rearend. Also shortly after production of the 8 7/8 MP rear from Pontiac ceased Ford came out with the very similar 8.8.

2) Is it possible (since the install kit I got had parts that were clearly Ford parts) that some of the parts may interchange between the 2? They appear very similar. The Ford 8.8 has tremendous aftermarket support while the Pontiac 8 7/8 MP has none. In fact Eaton lists nothing regarding the 8 7/8 MP.

fireprix, seriously doubt Pontiac or GM gave Ford all the Engineering Specs on the 8 7/8 MP series rear. The time frame of the RAV heads development was around '67-68, they were basically a copy of the Ford Tunnelport heads. Ford didn't introduce it's 8.8 rear til nearly 15 years later.

Upon the desire to replace the Ford 9" rear, am sure Ford's drivetrain engineers were aware of GM designs, & they most likely designed the 8.8, not necessarily with a clean sheet of paper, but using features in earlier rear designs which held up well. The Chevy 12 bolt car, Chevy 12 bolt truck, 65-70 Pontiac P series! & the 8 7/8 MP series, all carry a 12 bolt ring gear that is 8 7/8" in diameter. Torque output wise, the GM vehicles in the mid 80's were so anemic, I'm surprised the last rendition of the MP series rear was used as long as it was. An 8.5 B series rear was used under several heavy early 80's Buicks, then the 8.5 B body was reintroduced under redesigned Roadmasster wagons & Caprices with considerably more power than what Buick had in front of those last MP series rears.

Re:2) not sure what supplier put together the small parts order for you, but if a company like Ratech has labeled certain shims, as Ford 8.8, then noted they will also work in the 8 7/8 MP series rear, then more power to them. The carrier bearings/races used to fit a 30 spline Eaton 8.5 carrier into a previous 28 spline carrier design '71- early 1988 GM 8.5 housing, those exact carrier bearings & races were used in one design of 8.8 Ford rear. There are many other examples of small parts can be used in other mfg'ers rearend designs.

Been many times I wished the '71 up MP series rear had more aftermarket support, have chased & pulled my share of the late mid ratio 8 7/8 MP series carriers to convert to 31 spline side gears & use in P series builds. Unfortunately, the 8 7/8 MP rear design is not very popular. One reason i believe is it was never used in a relatively narrow housing application where it could be relatively easily swapped into smaller & mid sized chassis. Thankfully, years ago Brad at FabCraft had several ratios of gears made for the '65-70 P series axle & they had crossover into the '71-76 MP series rear & many Pontiac guys got on the early lists, bought up sets, & Brad was able to have more ratios of very hard to find gears made.

[Doug]

And, What about the 4 1/2 inch lug bolt pattern on the early Tempests? The ONLY GM car to use a Ford pattern.

[OG68]

Originally Posted by unruhjonny

1969 through to at least 1974 the three speed was a Muncie 330;
I do not know what GM decided to do after they stopped using this three speed.
It is entirely possible that GM did an about face and went to another supplier rather than manufacture them in-house, as this is precisely what they did with the four speeds found in cars.

Ford sold the rights of the three Spd top loader to Tremac in the mid seventies. Chevrolet then used it for their 76-81 trucks.

TREMEC T160 1976-81 Trucks
Cast iron, top-loaded case with side shift linkage. Fully synchronized with 36 tooth synchro rings. Same unit as Ford RAT except for a GM bolt pattern to the bell housing. Has R28-20-15 cluster gear. Found in heavy-duty applications.

When looking for a rebuild kit for my Dearborn I ran across the different applications for the Toploader 3 spd. I was surprised to learn that it was used for quite a few manufacturers besides Ford.
Tremac added an OD to the four speed version creating a three speed/OD. Used for light duty applications during the eighties.

[unruhjonny]

/\ thank you for sharing that!