# 12 bolt under 65 gto



## Dunnburger (Dec 18, 2011)

Currently have an open 10 bolt with 3:70ish gears under my 65. There is a 12 bolt posi rear available to me if I wish to purchase for a 66-67 gto (seller claims 3:42 gears) at what seems to be a good price. 

Would that fit okay? Would the 66-67 rear be wider? If so, too wide? I have rally II's on the car now and am not looking for fitment problems with the rear wheel openings if I were to buy the 12 bolt.

Or am I better off just turning my 10 bolt into a posi with 3:55's 

Eventually the motor will be a 461 stroker with HP north of 400 and torque north of 500 and a 5 speed tremec

your thoughts?

Josh


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## 05GTO (Oct 6, 2004)

Dunnburger said:


> Currently have an open 10 bolt with 3:70ish gears under my 65. There is a 12 bolt posi rear available to me if I wish to purchase for a 66-67 gto (seller claims 3:42 gears) at what seems to be a good price.
> 
> Would that fit okay? Would the 66-67 rear be wider? If so, too wide? I have rally II's on the car now and am not looking for fitment problems with the rear wheel openings if I were to buy the 12 bolt.
> 
> ...


Josh,

Just a couple clarifications, 65 didn't have a rear end with a 3:70ish gear, they had a 3.55 (open and Posi) and a 3.90 (Posi only), the 66 and 67 GTO did not have a 12 bolt. Pontiac started using the chevy 12 bolts in 70 for cars ordered with the 455.

If the rear end is from a 66 or 67 chevelle it should work under your 65 GTO as long as the spring and spring plate on the rear tube match.

The 12 bolt is a stronger rear end and will hold up better than the BOP 10 bolt for high HP applications.

Get the codes from the 12 bolt and I'll help you ID what the seller has available.


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## Dunnburger (Dec 18, 2011)

Thanks for the info/clarification! yeah the previous owner of mine said he put "3:70's" in it few years back. I need to just jack up the rear and spin the shaft to confirm but haven't yet. I figured a 12 bolt is probably what I'm gonna want to go with for my hp/tq (reallywant to go with a built 9 inch but the monies arent there for that just quite yet) In his listing for that 12 bolt he listed as a BOP a-body rear. I just naturally thought "oh GTO". I plan to give him a call tomorrow and I will get whatever numbers he can provide. 
If that falls through and doesn't work out, then I will probably just focus on building my 10 bolt. Hopefully I can find a cheap posi for the 8.2 and different gears. Run that until she can't run any longer. 

I'm also probably putting the cart before the horse as well. Im hoping to get the motor squared away first within the next 3-4 months. This 12 bolt came up at a good price, with posi and decent ger ratio thought I might want to jump on it. 

I will get back with some numbers after I talk to the guy

Thanks!


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## BearGFR (Aug 25, 2008)

When you look at that 12-bolt, make sure it's a real 12-bolt and not just a "Type O" 10-bolt out of an Oldsmobile (they had 12 bolt rear covers). Google is your friend.

Bear


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## 05GTO (Oct 6, 2004)

BearGFR said:


> When you look at that 12-bolt, make sure it's a real 12-bolt and not just a "Type O" 10-bolt out of an Oldsmobile (they had 12 bolt rear covers). Google is your friend.
> 
> Bear


:agree

If he indeed called it a BOP 12 bolt you would be better off with your current 10 bolt rear end. 

Chevy 12 Bolt










Oldsmobile 12 bolt


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## leeklm (Mar 11, 2012)

:agree

Oh yeah, been there done that!! One thing about the Type O diff is that it is a stronger 8.5" ring gear when compared to the 8.2" chevy 10 bolt. The Type O rear assembly is noticeably heavier than the 8.2".

I have a Type O 8.5" with 3.90 open diff if anyone is interested!


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## JustAl (Mar 20, 2010)

Very true what all have said. Any 12 bolt (Chevy type) will fit, however the width changed in 1968 and later, adding about 1/2 inch per side. Back spacing of the wheels can make these work as well. The spring pad also is different on the later rear ends requiring a rear spring from that year, chassis ends are the same .


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## Dunnburger (Dec 18, 2011)

Thanks guys for the info. Yep it's the olds 12 bolt not Chevy. Have done some thinkin. Originally I wanted a 9 inch to stick under there anyway. Now I figured I will suck it up, spend the little extra and go for one. I can get a 9 inch housing set of for and a-body with Axel's for $745 from "quick performance". A friend if mine has an extra third member and 4 different sets if gears that he will let me "borrow" for a bit.


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## BearGFR (Aug 25, 2008)

That was close --- that type O "12-bolt" isn't really a 12 bolt. If you pull that cover you'll find only 10 bolts holding the ring gear to the carrier, not 12. They aren't any stronger than the regular GM 10-bolt axle. Because they're something of an oddity now, they're extremly hard to find parts for.

Bear


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