# 10 bolt vs 12 bolt



## pcolenutt (Jun 6, 2013)

What is the technical difference between a 10 or 12 bolt diff apart from number of bolts? And what's a posi?


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## ALKYGTO (Mar 29, 2010)

10 bolt is factory 8.2 inch ring gear 10 bolts hold the ring gear to the differential. 12 bolt is the "C" or Chevy type rear with an 8.875" ring gear and 12 bolts to hold the ring gear to the diff. No parts interchange and the 12 bolt is stronger but since GTO's and Chevelles share the same chassis the two rears be swapped as an assembly and will fit. 1970 GTO's with the 455 used the Chevy 12 bolt, all other GTO's use the 10 bolt, even 4 speed cars.

"Posi" is GM's name for a "positive traction" differential where both tires are hooked together with a clutch type diff. Basically you had an "open" type rear where only one tire applies power to the ground or a "posi" where both tires put the power down.


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## pcolenutt (Jun 6, 2013)

Great info, many thanks.


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## 05GTO (Oct 6, 2004)

Also, watch out for the type-o (olds) 12 bolt which has a 8.3 10 bolt ring with a 12 bolt cover.

The 12-bolt type-c is available in 3 series, 

Series-2 uses ring and pinions that range from 2.29:1 to 2.73:1, Series-3 carrier uses ring and pinions that ranged from 3.08:1 to 3.73:1, Series-4 carrier uses ring and pinions ranging from 3.90:1 to 6.14:1


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## AMT1379 (May 11, 2010)

My 67 has a 12-bolt from a later model GM. Looks like there were some mods to make it fit. Overall happy with the performance of the rear from a strength perspective.

I am in the process of swapping out the carrier and gears now. It did have the 4.11s as expected, and that combo with the th400 and 400 ci engine have was NOT working out well. Made the car basically un-drivable at highway speeds. I just found out the clutches on the posi were shot as well. I'm dropping to the 3.31 and will report out on how it works out.


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## GtoGreene16 (Aug 20, 2013)

That's true about the 1970 gtos with the 455 having the 12 bolt I have a blue 455 conv. Ram air car and it has a 12 bolt from the factory.


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## pcolenutt (Jun 6, 2013)

My 455ci 4 speed hurst 67 gto has a 10 bolt running 3.73 gears on a 25" diameter tyre (225/70R14), so same issues - highway was intolerable, gave me headache, seriously. Looking at 3.08.


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## kjk990 (Sep 1, 2010)

I have a 67 GTO with a TH400, 2.56 (I think) I wanted more power off the line. So I added an Eaton limited slip dif and 3.55's. I loved it off the line but on the freeway my engine was screaming. At 65 I was at 3200 RPM. Big trucks were passing me.

I replaced the TH400 with a 700R4. I love it. It may be the best upgrade I made to the car. First gear is lower than the TH400 and I have an over drive gear and a lock up torque converter. Now I am at 1900 RPM at 65 MPH.

I pulled the 700R4 from Pick N Pull for $100. 
$600 to rebuild. 
$100 for adapter plate. 
$200 for lock up torque converter.
$250 for a new drive shaft.
$100-$150 for misc parts.

total $1350-$1400 

Changing the gears was about $400. I drive my goat every non-raining weekend. I love it.


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## geeteeohguy (Feb 2, 2008)

As I said on the rear gear thread, 3.08's behind a 455 and a 4 speed in a '67 GTO will be an excellent choice for all around driving. Decent fuel mileage, too, if you keep your foot out of it. You will LOVE the punch it has from 30--120 mph! And you'll be right on the power band at 70-80mph, instead of running out of steam there.


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## pcolenutt (Jun 6, 2013)

Great, thanks guys. So tell me, where does "Goat" come from?! Only just seen this!


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## silversport (Mar 23, 2007)

mixing the letters GTO...(GOaT)

Bill


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