12-Bolt Question

[C15_Sierra]

I bought my '80 Sierra with a 305/TH350 and 2.73 gears (yea, old man's truck). The motor and tranny have been replaced already with a 5.3/4L60E so it's time to focus on the other bits to spice up the truck. One of the things I'd like to do is get rid of the 2.73 gears (who wouldn't) and put a posi-trac out back. I'm thinking 3.73's since I have OD now.

So my question is...did the PO at some point swap out the rear axle? I don't know much about 12 bolts, but I've read that 2.73 came in passenger cars but not trucks. FYI I measured out the # of driveshaft rotations per wheel rotation (2 and roughly 3/4 = 2.73). Is there anyway to verify if I have a car or truck 12-bolt?

And my next question is, assuming I figure out the first one, is in order to swap to a posi-trac, do I have to replace anything else other than the carrier and gears?

Thanks!

[jimboh247]

Just going by shear numbers, chances are, it's a truck 12 bolt.

The majority of the 12 bolts in cars were the 4 link/coil spring variety found in Chevelles/Impalas(different width) and such. Literally a gazillion of those out there.

There are not nearly as many leaf spring 12 bolts floating around. They came in early Z-28's, big block Novas and the like. Very desirable! The vast majority of Camaros and Novas came with the 8.5 10 bolt.

I am thinking the truck 12 bolt had a different size ring gear than the car 12 bolt.

As far as switching to a posi rear end, as far as I know, you only need the proper carrier, the gear set, and the approperiate bearing set.

[craig@ridetech.com]

FWIW, my 76 has a factory 12 bolt rear end in it. I don't know the gear ratio off the top of my head. The truck was a facotry I6 with a 3spd. I converted it over to a small block with TH350, didn't touch the rear end at all, still drives great and cruises pretty well at 65.

[C15_Sierra]

Mine also was a factory I6/3spd truck that was converted to a 305/TH350 in 1988 I believe. The engine and trans were out of a '79 Camaro. It was my understanding that the smaller-engined trucks were geared numerically higher to get the most out of the engine, while the V8s had the torque to overcome taller gears

I'll take pictures this weekend to help clarify, thanks for the responses!

[hotrod_991]

I have a 77 350/TH350 and it is a factory 12bolt with 3:73's and a factory GM Gov-Lok

Correct me if I am wrong but I think the truck 12bolt had thicker housing trusses. Metal added from the pumpkin to the tubes to strengthen it.

[veinbar]

Correct me if I'm wrong... There is no gear lower than 3.08 for GM 12 bolt Truck rear ends. I am in this position myself. I have a 12 bolt in my '79 SWB that is in serious need of a rebuild or replace. After looking at every vendor I can think of I can only find ring and pinions in the following sizes:


3.08... 40-13 (What I currently have)
3.42... 41-12
3.73... 41-11
4.11... 39-9
4.56... 41-9

[veinbar]

I vote 3.73 posi rear. That will put you around 2200 rpm @ 70 (depending on tire size). For a 5.3L you'll be in a very comfortable rpm and should produce impressive efficiency. When you have your rear rebuilt, please tell me how much you're charged. I'm shopping around for rebuilds now, but they are expensive.

Thank you, and I'd love to hear about your mpg when all is said and done.

[Maxxd07]

I have a 79 4x4 that had 2.76 gears with a 350 4 spd combo in a 12 bolt rear 10 bolt front. I also have an 80 4x4 with 3.08s and a 350/350 combo also 12 bolt rear 10 bolt front. I converted the 2.76s to 3.73s with a thick gear from yukon since a different carrier would be needed otherwise. 3.08s and numerically lower had a different carrier than the numericlly higher gears. Also the truck 12 bolt has a 8.75 ring gear vs 8.5 of a car.
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[C15_Sierra]

Thanks for the help you guys...

Veinbar, this was the first time in a while (3 mos.?) since I've signed into this site...I've since moved to Hartford, CT which is part of the reason I haven't touched the truck (or this site for that matter) in 2 months. I wasn't planning on rebuilding the rear, just replacing the carrier and gears myself and learning as I go. As far as my driveshaft, I was going to get it shortened at Inland Truck Parts near 35 and 183 because it came highly recommended by people at work.

[LONGHAIR]

Originally Posted by C15_Sierra

I wasn't planning on rebuilding the rear, just replacing the carrier and gears myself and learning as I go.

In a word....."don't"

First of all, gear swaps are not a simple bolt-in. It is a very precise operation that is best done by an experienced "Professional"....and since the bearings have to come off anyway, you might as well go through everything.

[C15_Sierra]

Originally Posted by LONGHAIR

In a word....."don't"

First of all, gear swaps are not a simple bolt-in. It is a very precise operation that is best done by an experienced "Professional"....and since the bearings have to come off anyway, you might as well go through everything.

That's good to know, thanks. I have some friends (one of whom is a former tech who has rebuilt the rear end of his SVT Cobra) who are willing to help out so I won't be completely alone.

Got other things to accomplish before I get to the rear end though, I was mainly asking these q's so I can start to gather parts

[ghettoluxury]

Originally Posted by veinbar

Correct me if I'm wrong... There is no gear lower than 3.08 for GM 12 bolt Truck rear ends. I am in this position myself. I have a 12 bolt in my '79 SWB that is in serious need of a rebuild or replace. After looking at every vendor I can think of I can only find ring and pinions in the following sizes:


3.08... 40-13 (What I currently have)
3.42... 41-12
3.73... 41-11
4.11... 39-9
4.56... 41-9

Here is your correction my 79 has factory 2.76 in it's 12 bolt

[veinbar]

Originally Posted by ghettoluxury

Here is your correction my 79 has factory 2.76 in it's 12 bolt

That's good to know. My confusion stems from aftermarket sizing availability. I haven't found a company that sells lower gears (numerically) than 3.08. If you find a vendor that does, let me know.

That gear must be great on the highway? My buddy has a van with that gear and he sits at 2200 or so rpm @ 70mph. Must be nice. I'm ramped up to 2500 to maintain 65...

[ghettoluxury]

It is awesome for the highway but sucks in town.

[veinbar]

Originally Posted by ghettoluxury

It is awesome for the highway but sucks in town.

Sure... In town mileage for all of us isn't that impressive to begin with. I would imagine that first gear is tragically weak/slow off the line. 3.73 w/ a 700r4 seems to be the magic combo. This is my "want to" build for my '79. $$$$ is the deciding factor at this point.

[Pyrotechnic]

RPM drop is not everything. My 77 came with 3.42 gears in it, and it got about 14/15 city and highway mileage. That 3000 RPM at 65 MPH

Since I had a 12 bolt from my old truck, with a 3.08 and posi that was totally rebuilt I swapped that in. Guess what? It got the same 12 MPG no matter what kind of mileage that my old truck got. It wasn't nearly as fun to drive, and on top of that, it seemed to run hotter all the time. Everything was kept constant, except the rear end gears.

By lessening the mechanical advantage that the motor had over the tires, it like towing a trailer all the time.

Also keep in mind, 700R4's have a converter lockup to help mileage but it will unlock to help you accelerate, or if you mash the pedal enough it will downshift to 3rd.

With highway gears, you don't have any of that. The motor just fights the gears and slips the converter all the time. This become a factor when there are hills, or you have to slow down or pass someone when shuffling through traffic. Unless you get to run on flat roads all the time with no traffic, a highway gear might hurt more than it helps.