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Post Info TOPIC: 65-66 Pontiac 12 bolt diffs


A Poncho Legend!

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65-66 Pontiac 12 bolt diffs


In the past we've had some discussion about which cars came with 12 bolts and why. There not be any new info in here for anyone reading this because of previous discussions about it but I'm hoping someone might have some new info.

Some people theorize at least 327 cubic inches, others say at least a V8 and some say any engine could have been had with a 12 bolt. Some say the car has to have F40 HD suspension. Some say there was a shortage of 10 bolt diffs during production and all the cars got 12 bolts for a while. 

This 6 cylinder 66 station wagon I dragged home on the trailer yesterday has a 12 bolt. I decided to try to find the stamping on the axle tube but with the car on the trailer it's difficult. I've been under there scraping and buffing and taking pictures and washing it with brakecleaner but so far no code to be found. 

Plan B was to check the sales literature to see what ratio it could be.  

6 cylinder wagons with 3 speed manual or Powerglide all came with 3.55. Ok, that may well be because 10 and 12 bolt diffs both offered 3.55 ratios in these cars. 

So on to the other wagon I bought. It's a 66 also, a 283 Powerglide, also with a 12 bolt. General purpose ratio as listed in the sales literature says 3.36 for 283 wagons. This is where I run into a snag. 3.36 is only a 10 bolt ratio. Also, for Strato Chief sedans with 283, 3.08 is the General purpose ratio. My Strato Chief sedan is a 283 with a 12 bolt 3.07 ratio from the factory. I know that rearend was installed on the assembly line.  So for sure the brochure is wrong about 6 cylinders and 283's.

So now that I've bored you with details, I'm starting to think the theory about the shortage of 10 bolt diffs, or else just a change in thinking occurred. Maybe the decision was made to install 12 bolt diffs in many or all 6 cylinder and 283 cars, at least for a while because when my sales brochure was printed (Sept 1965) there was no hint of 12 bolt diffs in 6 cylinder or 283 cars, even including wagons. 

Does anyone have a 66 sales brochure that was published later in the model year? The date of publication is printed on the bottom of the back page. 

t1.jpg



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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)



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And would any rear end be available as an option on an order for a new car?

I assume that is not true of the 12 bolt in your 66 sedan which you would have GM paperwork for, it is not listed as an option?

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63 Parisienne sport coupe (The Big GTO), black, maroon interior, 409 4 speed; former owner of a 59 El Camino, 63 Corvette SWC, 62 Chev Bel Air SC.
1963- Pontiac top selling car in Canada

Mahone Bay, NS Still not old enough to need an automatic



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Interesting info for these. Mine originally came with a 283 PG and 10 bolt, 3 link. I replaced it with a 12 bolt that had 3:36 gears only I don't know if they were original. Also cleaned up the entire unit and never found a stamped code anywhere. I later changed to 4:42, which worked much better.



-- Edited by 1965CS on Wednesday 29th of May 2024 07:06:51 AM

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DonSSDD wrote:

And would any rear end be available as an option on an order for a new car?

I assume that is not true of the 12 bolt in your 66 sedan which you would have GM paperwork for, it is not listed as an option?


 No, there is nothing in my option list denoting anything out of the ordinary for the diff on my Strato Chief 2 door. I have a copy of the order guide with options and the costs for the options on a 66, no mention of being able to order an HD diff. Nothing other than being able to order optional gear ratios on certain models. And that coincides with the yellow chart listing "Special Purpose" and high performance.

And now this morning I checked the order guide that Bob Crocker so graciously posted here many years ago, which I saved in the 65-70 forum. It doesn't seem to agree with the sales brochure. Back to the drawing board!

The Strato Chief wagon is a model 75335. The other car (which I don't have here yet) is Parsienne, 76435.

 1966 Pontiac Order Guide, Options, Prices Etc. - Canadian Poncho (activeboard.com)



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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)



Poncho Master!

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My experience is mainly 65 Chevys. All the 283 PG cars I have had anything to do with were all 10 bolt 3 link including 1 65 Custom Sport . The 1 65 Belair wagon was a 6 3sp and had a 12 bolt 4 link 3:55 gear.



-- Edited by DANO65 on Wednesday 29th of May 2024 09:39:50 AM

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My 65 stratochief wagon is 12 bolt 3 on the tree inline 6 , no markings on diff ,

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A Poncho Legend!

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All I found on the diff so far was B B which makes no sense for a 66 12 bolt code but I marked the tire and driveshaft, spun it, exactly 3.55 just like the brochure says.

Crazy, a 12 bolt with nothing on it that makes sense. I'm going to pull the diff eventually and then it's going to get the cleaning of a lifetime to see if it's stamped.

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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)



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This thread reminds me of a 1965 Biscayne I had many years ago, 6 cyl with a 12 bolt.

I've looked under many cars over the years just to see if they might have a 12 bolt.

Why I don't know, but 12 bolts certainly have an allure only a car guy understands.

I've never seen a 68-70 Nova SS 350 with a 10 bolt but read automatics got them.

It seems Canadian cars are more likely to get a 12 bolt under odd circumstances??

Didn't some East coast built 70-71 GTO's come with a Chevy 12 bolt??

 

Thanks

Randy



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GLHS60 wrote:

This thread reminds me of a 1965 Biscayne I had many years ago, 6 cyl with a 12 bolt.

Yes, this isn't the first 6 cylinder 12 bolt car I've seen. And the US guys think we're nuts if we mention it. As far as I've ever seen, they never had that combination.

I've looked under many cars over the years just to see if they might have a 12 bolt.

They could be under anything. Don't quit looking.

Why I don't know, but 12 bolts certainly have an allure only a car guy understands.

Absolutely! Anyone who's seen broken 10 bolt diffs looks for them........!

I've never seen a 68-70 Nova SS 350 with a 10 bolt but read automatics got them.

It seems Canadian cars are more likely to get a 12 bolt under odd circumstances??

 I totally agree with you. I had heard the theory that they ran out of or ran low on 10 bolts during production but this car is a very early car, so that would seem less possible? Unless there was a supply issue? Some 10 bolt components maybe?

Didn't some East coast built 70-71 GTO's come with a Chevy 12 bolt??

I think again, the Canadian cars had some 12 bolts that the US cars didn't.

Thanks

Randy


 



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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)



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Meant to say some East Coast built 70-71 GTO's built and sold in the US.

Thanks
Randy



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I plan to check the parts book when I get a minute. I think there is a notation in there when you look up parts that says "with 12 bolt cover Canadian Jobs " or something to that effect. And I'm pretty sure it's in the 70-72 era; just can't recall exactly which years.

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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)



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RPO Z04 - Heavy Duty Chassis Equipment. Includes h.d. springs & shocks (F40), front stabilizer bar (F32?) which is normally deleted on 6-cyl Biscayne & Bel Air sedans even with F40, h.d. rear axle (12-bolt), h.d. brake webbing, hardware & shoes (J65). Z04 is offered on 153-154 Biscaynes & 753-754 series Strato Chiefs. No a/c or disk brakes allowed. Z04 is also included as part of the B02 Police & B07 Taxi options. The B02 Police option offers many exceptions to the normal ordering rules.

Heavy Duty axles that come with the 12-bolt are part of the arsenal for severe service like taxi & police use. Potholes & bashing, washboard roads are killer on axle shafts. It isn't all just about clutch-dumping a 375-horse engine. You'd get stronger housings, bearings, shafts, ring gear, pinion, and U-joints. With a semi-floating axle, when it breaks you lose the wheel. Live axle cars (i.e solid axle) are all semi-floaters. Full-floaters are 1-ton and up trucks, plus much older 3/4-ton trucks.


Way back GM Canada did special Buicks for Newfoundland & Labrador with all it's punishing roads. Numerous unseen upgrades to address familiar failures from roads that make Yonge Street & the Baja look easy. Maybe like the Alaskan Highway, only way skinnier & they call you "Boody" when you finally pull into a cove by the "toid" (sic). wink



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67 Chevelle Malibu Sport Coupe, Oshawa-built 250 PG never disturbed.

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All australian chev/poncho cars have 12bolts with F40 suspension package. 



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cutting a roof off a four door is NOT a convertible.....

65 Parisienne convertible.one of 49 built for RHD export market,402BBC, T400, 2500 stally, posi rear, upgraded brakes with front discs, FUEL FAST efi custom built by me.



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GLHS60 wrote:

Meant to say some East Coast built 70-71 GTO's built and sold in the US.

Thanks
Randy


The 12-bolt was an option in the US only in a 1970 Pontiac A-body equipped with the 455-HO engine option. The GTO Judge would have been the common place to find one.

 



-- Edited by gtodrive on Sunday 2nd of June 2024 08:46:02 PM

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A Poncho Legend!

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I figured it was time to dig out the parts book for the Canadian A body Pontiacs. It is kind of as I remembered. We had 12 bolts in them from 70-72, depending on what was ordered. For example, in the 80's I worked at the Pontiac dealership in town for 5 years. During the time I was there, my boss bought a really nice 53,000 mile original paint 72 GTO 455 automatic. It had a 12 bolt posi, as I recall 3.31 ratio. 

It's crazy how many times we have seen Canadian GM cars differ from the identical model in the US with options, equipment etc.  

 

20240602_181040[1].jpg



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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)



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My son has a 1969 Beaumont Station Wagon that came from the factory with a 250 cu. in. 6 cyl., 350 Turbo trans. and a 12 Bolt rear end. Radio delete, Rubber flooring, the 2 way tailgate, Vinal seating and Power Steering all on the Basic Beaumont. I thought it weird that it had a 12 Bolt Diff. but maybe all A Body Wagons came with a 12 Bolt ? Who Knows ?

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Certainly in Canada GM was quite liberal with their 12 bolts vs the US cars. My brother had  a 69 Cutlass 2 door hardtop 6 auto with a 12 bolt. That one sure never made sense.

But the car had power windows too! Go figure on an otherwise very plain car.



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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)



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The B body Chevs are the same for the most parts. Guys I've talked to say there seems to be more 12 bolt than 10 bolt and very common on low HP cars, 327 and the 6 cylinders. They seem to show up even when not checked on the option sheet, no charge. But check the box and you get charged, makes no sense.

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It has to simply be a case of supply and demand. Wouldn't it be nice to actually get the real story from a GM insider?



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Here's one just posted on FB under Canadian Pontiacs. A 65 4 dr with 6cyl auto and has a 12 bolt.



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1965CS wrote:

Here's one just posted on FB under Canadian Pontiacs. A 65 4 dr with 6cyl auto and has a 12 bolt.


 I've had guys look at me a bit funny when I told them my uncle bought a brand new 65 Laurentian 6 cylinder 3 on the tree with a 12 bolt posi. He lived on the farm, wanted the posi due to mud and snow sometimes being an issue.



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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)



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The Canadian-built LeMans 1970-72 would use a Type C 12-bolt for instances where in the U.S. they would use an 8.5" 10-bolt (e.g. like 400s and 350s with with manual transmissions, such as GTs and floor-shift cars.). The 455 cars all used Type C 12-bolts, Canada or U.S.A.

455 L75s were automatic only and came with 3.31 by default (a/c got 3.07). The 455 H.O. LS1 used 3.55:1 gearing, which is fairly high for a long-stoke 455.

Usually Canadian LeMans with orange 350s used Chevrolet 8.2" 10-bolts, but during 1970-72, like Chevelles, they received Type C 12-bolts when F40 suspension was factory ordered.

If you look for consistency or logic you won't find it. In 1965 all 327 cars got 12-bolts but in 1966 none did except for L79s. Rarely if ever did stuff get randomly installed on the assembly line. One exception was 3.55:1 gearing in Chevelles or Camaros as you'd get a 12-bolt almost always.


Oldsmobiles were special. They were known for strong axles, and not being a value-brand like Chevrolet often mean for a few bucks more on the base price you'd get little things like that included. Type O 12-bolt with bolt-in axles in the U.S, Type C with 12-bolt with press-in axles in Canada.

Buick gave better brakes & cooling as base. Kinda like VW vs. Audi today with the brakes.

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67 Chevelle Malibu Sport Coupe, Oshawa-built 250 PG never disturbed.

In garage, 296 cid inline six & TH350...

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DD, 3.31 in my '66-Laurentian 9-passenger. Not sure of the engine other than a Mouse. I may do the docs on it. I have a '65 Parisienne Wagon here as well. I'll try and see what's in that one.

I do have a '70 2+2 here with a 10 bolt posi. Was a 350 car.. that one is odd. 

That water logged '65 Strato with the 230 and a 3-ala-tree. It was a 12 bolt. My '66 Chief 4-dr sedan was a 250, 3-ala-tree and a 12 bolt.

My 1st '67 Grande 4-dr h/t ws a 327 'glide and 10 bolt originally.

IMG_0315.jpeg





-- Edited by 67Poncho on Saturday 8th of June 2024 11:51:39 AM

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Vincent Jr.



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The D D makes sense with that yellow chart I posted too. 

Do you happen to know if the 66 sedan is 3.55 and if so, what the code is?



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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles 

1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)



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I had an interesting day diff searching and decoding. I've had my GM book since early '90's and there is small print that I only read yesterday that was brought to my attention by a fellow member Cheviac. I have always found the axle codes on the front side of the right tube but as you read, they say it is supposed to be at the bottom of the tube.

Also learned about axle identification when they contain a posi unit. I looked for the identification on the 1965 wagon, but only scratch the front side and now I have to go back and check the bottom of the tube as well. 

Another oddity, only '65-'67 10-bolt axles had the 3-arm set up. The '68 thru '70 charts don't have that listing as an option. The 10-bolt in the '70 2+2 I checked has 4-arms.


 IMG_2987.jpeg
IMG_0330.jpeg

IMG_0357.jpeg

IMG_0348.jpeg

IMG_0327.jpeg

 



-- Edited by 67Poncho on Sunday 9th of June 2024 12:47:10 PM



-- Edited by 67Poncho on Sunday 9th of June 2024 12:55:11 PM

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