# 12 bolt dif



## Vic66 (3 mo ago)

Hello all, 1st post of many to come. Picked up a 66 lemans vert, 1 owner 51k.








I had picked up a chevy 12 bolt to put in a 72 lemans i no longer have. That will not fit in my 66, will it?


----------



## geeteeohguy (Feb 2, 2008)

It should, if it fit in the '72 Lemans. Nice looking car!! In all the years I've been doing this, I've found that the cars with the fake wire wheel hubcaps were usually purchased by older folks and if they still have them on the car decades later, the cars always seem to be cream-puffs. Your appears that way.


----------



## Duff (Jan 12, 2020)

Yes, it'll bolt right in your 66, the DS upper control arm may or may not hit the 12 bolt's housing, if it does, EZ fix, just grind maybe 3/8 off the end of the control arm, don't touch the housing. Pontiac Jim did a write up on this a while back. Nice upgrade over the stock 10 bolt!


----------



## 1969GPSJ (Feb 26, 2020)

I would think it should , just to be on the safe side I would measure the width at the backing plates of current rear to confirm 
Nice looking Vert 
is that CA black tag?


----------



## Mine'sa66 (Oct 30, 2019)

It will bolt right in, Depending on what year it is, if it's a later one (late 67&up), your track will be about 3/8 inch wider.


----------



## GTOJUNIOR (Aug 7, 2011)

What's wrong with the 10 Bolt?


----------



## PontiacJim (Dec 29, 2012)

Vic66 said:


> Hello all, 1st post of many to come. Picked up a 66 lemans vert, 1 owner 51k.
> View attachment 159972
> 
> I had picked up a chevy 12 bolt to put in a 72 lemans i no longer have. That will not fit in my 66, will it?



Read this thread:









12-Bolt Chevy Rear End Into A Pontiac GTO


Chevy A-body 12-bolt in an A-body Pontiac. Posting this info/swap as this question comes up often enough. I put together what info I could find, but there could be more that I missed. The big thing is the driver's side upper control arm has to be modified for the 12-bolt installation...




www.gtoforum.com


----------



## 67ventwindow (Mar 3, 2020)

Welcome, Can't wait to see more pictures.


----------



## Vic66 (3 mo ago)

1969GPSJ said:


> I would think it should , just to be on the safe side I would measure the width at the backing plates of current rear to confirm
> Nice looking Vert
> is that CA black tag?


No, picked it up in Delaware. Got the protect plate! Needs a little bit of body work in all the usual places, all in all not bad. Just picked up a center console. Want to add the GTO hood and some rally 1's with redlines.


----------



## Vic66 (3 mo ago)

GTOJUNIOR said:


> What's wrong with the 10 Bolt?


Nothing, I have to check what the gears are, will pep it up a bit than what's there. Also just saw a listing for a GTO posi, with 3:55's, waiting to hear back on that. Also want to swap out the 2 spd for a TH350.


----------



## GTOJUNIOR (Aug 7, 2011)

That 10 bolt is more than adequate for the 326 if it's an AT it likely has 3:08s which are good HWY gears, 
just bumping up to 3:23 would be a very noticeable change.

IMHO I would sell the 12 bolt and buy gas. If it ain't broke as they say.
JM2C 
Cheers.


----------



## armyadarkness (Dec 7, 2020)

Vic66 said:


> Nothing, I have to check what the gears are, will pep it up a bit than what's there. Also just saw a listing for a GTO posi, with 3:55's, waiting to hear back on that. Also want to swap out the 2 spd for a TH350.


If you're by delaware, keep me posted. I can help with gears and a trans.


----------



## armyadarkness (Dec 7, 2020)

Agreed. Leave the BOP and sell the 12 bolt for parts money!


----------



## Vic66 (3 mo ago)

armyadarkness said:


> If you're by delaware, keep me posted. I can help with gears and a trans.


I'm up in CT


----------



## Vic66 (3 mo ago)

Vic66 said:


> I'm up in CT





Vic66 said:


> I'm up in CT


Appreciate the offer!!!


----------



## armyadarkness (Dec 7, 2020)

Vic66 said:


> I'm up in CT


I cruise in CT a lot and go to the race track there, but yeah, it's too far to go for diff swapping!

Changing the gears in a BOP isnt very difficult.


----------



## Vic66 (3 mo ago)

armyadarkness said:


> I cruise in CT a lot and go to the race track there, but yeah, it's too far to go for diff swapping!
> 
> Changing the gears in a BOP isnt very difficult.


I actually have a set, would have to see if they fit, already forgot what they are, lol


----------



## armyadarkness (Dec 7, 2020)

Vic66 said:


> I actually have a set, would have to see if they fit, already forgot what they are, lol


Yeah, theres not much point if swapping out gears, without first knowing why youre doing it.


----------



## armyadarkness (Dec 7, 2020)

MSD Dizzys and handles for shifters,
Lunati cams that are bundled with lifters,
Oval port heads that come complete with springs, 
These are a few of my favorite things.


----------



## Scott06 (May 6, 2020)

GTOJUNIOR said:


> That 10 bolt is more than adequate for the 326 if it's an AT it likely has 3:08s which are good HWY gears,
> just bumping up to 3:23 would be a very noticeable change.
> 
> IMHO I would sell the 12 bolt and buy gas. If it ain't broke as they say.
> ...


This is good advice unless you are hammering it no need to put in the 12 bolt. 

Agreed on the 3.08's -I went from the 4.33's the PO put in my car to 3.08s, and I wish I had done a 3.23, 3.36, or 3.55. Its nice to cruise in but makes the car pretty tame. 

As stated the 8.2 is pretty easy to swap gears in. I had the 3.08s in a complete rear out of a 66 Cutlass from a u pull it yard ($45 in 1993/94). With a little patience its not a hard project. I bought a second pinion bearing and hogged out the ID of it with a flap wheel so it slides on and off the pinion gear made shimming/setting up the pattern easy...buy and extra crush sleeve or two.


----------



## PontiacJim (Dec 29, 2012)

armyadarkness said:


> I cruise in CT a lot and go to the race track there, but yeah, it's too far to go for diff swapping!
> 
> Changing the gears in a BOP isnt very difficult.


What, no casino time?


----------



## armyadarkness (Dec 7, 2020)

PontiacJim said:


> What, no casino time?


Working in Atlantic City for the last 20 years, has cured me of any desire to gamble.

$20 at the Wheel of Fortune slot machine, is about as adventurous as I get. Although, I expect that the Connecticut casino hookers must be WAY better than the AC ones. They probably drive BMWs and dont have bullet wounds


----------



## PontiacJim (Dec 29, 2012)

armyadarkness said:


> Working in Atlantic City for the last 20 years, has cured me of any desire to gamble.
> 
> $20 at the Wheel of Fortune slot machine, is about as adventurous as I get. Although, I expect that the Connecticut casino hookers must be WAY better than the AC ones. They probably drive BMWs and dont have bullet wounds



Lambo's and they even smell nice - no flies either.


----------



## armyadarkness (Dec 7, 2020)

Me on my way to Connecticut...


----------



## Vic66 (3 mo ago)

Sound advice. My purpose is just to pep it up a bit, nothing crazy. I would be happy with 3:36/55. Swapping out the 2bl, for 4bl, was going to do headers, but now I'm leaning foe the ones from ram air, dual exhaust and th350. That's probably it for now. Just received today an auto console I picked up. Will be a work in progress.


----------



## hillpc (4 mo ago)

Vic66 said:


> Also want to swap out the 2 spd for a TH350.


From a gear ratio standpoint only, this is an excellent idea for waking up the low end torque delivered to the existing rear, without suffering any of the downsides (to highway mileage and excessive cruising rpms) of swapping out the rear axle ratio, since the final gear ratio of the TH350 is still 1.00, just like in the 2 speed Super Turbine 300 automatic that these cars came with. (My 66 GTO has the ST300, with a 3.23 rear gear, and it could stand some more torque multiplication off the line, but I'm not messing with it because it's a correct California survivor car.)

The Turbo 350 has a 2.52 first gear, vs the ST300 at 1.76. The (or some of the?) Buick and Olds ST300s were available with a "switch pitch" torque converter, which got better off the line acceleration. Maybe Pontiac thought the GTO 389 had enough torque (around 430 ft pounds) that they didn't need a lower gear or higher stall speed converter?


----------

