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Guys,

Anyone here know if GTOs ('66-'72) came standard with a 12 bolt like Big Block Chevelle Super Sports, or was the 12 bolt an option? Lately I've been looking around and have seen many Goats for sale that are advertised as being "original," but they'll sport a 10 bolt out back. Seems kind of strange.

Any help would be appreciated.

Thanks!
 

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I have a 67 GTO. The 10 bolt was standard equipment. I think in the later years, 12 may have been an option but the earlier years it was 10. Like the Chevelles, most were not posi, even tho people will tell you "It's not a real GTO with an open rear", just like I've had a guy tell me my 66 SS is not real because of the idiot lights, not gauges.
 

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The 12 bolt didn't show up in the GTO until 1970, and even then it only came with the 455. I've never heard anything about it being an option. If it's any consolation the BOP 10 bolts are beefier than the Chevys, and they don't use c-clips.
 

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Our '72 400 4 speed Lemans Sport came with the 3.55 Chevy 12 bolt posi rearend. It was Oshawa built if that makes any difference. I think that the '60s Pontiacs had the cone type Posi rather that the clutch packs that Chevy used.
 

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Our '72 400 4 speed Lemans Sport came with the 3.55 Chevy 12 bolt posi rearend. It was Oshawa built if that makes any difference. I think that the '60s Pontiacs had the cone type Posi rather that the clutch packs that Chevy used.
Keith, my neighbor had a very nice original '72 Lemans GT 400 4-speed that was built in Pontiac, Michigan. It had a corporate 10-bolt 3.55 Safe-T-Track. Since your Lemans was Oshawa-built, that is rather telling. I have also seen and heard of Canadian-built Oldsmobile Cutlasses using the 12-bolt Chevy rear rather than the tough Olds rear.

The 12-bolt rears, as was stated above by Mr. 67_GTO, wasn't used until 1970, and only behind the high-torque 455s. The 455 used a 3.31, a 3.07 with a/c; or a 3.55 12-bolt behind the 71/72 455 H.O.

Even the Ram-Air 400 cars through 1970 used 10-bolt corporate rears, sometimes with 3.90 or 4.33 gearing.
 

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I think a lot of guys get confused with the 12 bolt thinking its what all GM devisions or at least all A bodies recieve. The Chevy 12 bolt is different then the other GM 12 bolt rearends. I believe Olds also have a rearend with 12 bolts on the rear cover right? I do know that the Pontiac 10 bolt is a better rearend then the Chevy 10 bolt and is no reason to turn your nose up. They are not a bad posi rear at all actually.

Thats about all I really know about the BOP style rears really. Do they have an aftermarket following as far as axles and posi units go? Or are they suffered by the lack of aftermarket performance parts because everyone wants the Chevy 12 bolt. Those are really what im not sure about.
 

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Cam knows what he's talking about, some of the Canadian Oldsmobiles did indeed use the real deal 12 bolt. Not sure which ones, but I did have a Canadian built 68 442 (badly rusted, but I got it from the original owner) with a 3.07 non-posi. I've seen the Olds 12 bolt and it's definitely not something that's easily mistaken for the Chev once you know better.

Not sure what all is available for the BOP 8.2 nowadays, but I'm pretty sure that Richmond is making gears for them now, aftermarket axles shouldn't be too hard to get? And Eaton and Auburn offer posi units. Think they're limited to 28 spline, but I've also heard that some people have (with a little grinding?) managed to shoehorn an 8.5 unit into there.
 

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An Olds 12 bolt (O Type) is completely different than a Chevy 12 bolt ( C Type). An O type has 12 bolts holding the cover on, but only 10 bolts mounting the ring gear to the differential. It has an 8.50" ring gear. Most have 28 spline axles with the exception of units having 3.90-up gearing, they were 31 spline. The differential is a clutch style with an S spring. The axles have pressed on bearings with a retainer plate to hold them in place.

All in all, they will not hold up to the abuse like a C type will, not to mention parts availability is next to none.

I snapped an axle in mine last fall racing in Iowa. I sure am thankful for the bolt in axles.:hurray:

And yes, a Canadian built Cutlass/442 had C type units installed
 
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