GM 12 Bolt

Hey guys I have a 12 bolt rear from an A body sitting in my shop at the moment and I am thinking about sticking it under the Cutlass this summer. I picked up the rear cheap here local and it was complete drum to drum. It does have a leak so I figure I need to crack it open and get a new gasket in there at the very least. Currently it seems that the rear is an open 2.73, not exactly great for performance. From what I understand is this would make the rear a 2 series carrier meaning I could not swap up to a lower rear gear in the 3 range is that right?

It is possible to change the gear to something better than the 2.73 with this rear or should I just ditch it? I seem to remember something about being able to swap either S10 or F body guts into a stock G Body 7.5 10 bolt but I am not sure of the specifics there. Anyone know?

I've heard with other axles that you can switch the carriers to get into other ranges of gear ratios, but I don't know anything specific about a car GM 12 Bolt. Randy's Ring and Pinion has a lot a good info...

Another thing to consider, can you set up a rear end or do you know someone who can? Might be easier and cheaper to find the whole axle with the gear ratio you want...

The 2.73 carrier is a 3-series carrier, so any of the 3.xx gears should fit (and I think up to 4.10 as well). There are no other carriers that will directly fit in there, just the 12 bolt 8.875" carrier. You wouldn't want to use the wimpy 7.5/7.625" anyway.

What year is your Cutlass? The later A/G bodies won't take an early 12 bolt without modification. You may also find that the 2.73 isn't the end of the world depending on your torque. A 400 or 455 makes enough torque to get that (relatively) light car moving with 2.73s, but if you have OD it will bog down a bit on the highway and shift in and out of OD a lot.

In reply to curtis73: Are yo sure about that? I was waiting for someone to chime in here but I thought the 12 bolts had different series'. Maybe that was just 10 bolts... I am never sure.

If he is planning on entering the challenge, he'll definitely want to ditch the 2.73's. I had some too, and they're great for cruising, but with the dragstrip passes, you'll want something MUCH higher.

In reply to curtis73:

You sure its 3 series I coulda swore it was a 2?

Well the combo I will be using is a 403 that should be getting a new intake and headers this summer and hopefully high compression heads and a new cam next year. Oh and there is a nitrous kit being pieced together . The transmission is going to be a TH350 with shift kit. The car is a 1980 A/G body car so I know I will need to use the adjustable upper control arms with spherical ends to keep things from binding up.

tuna55 wrote: In reply to curtis73: Are yo sure about that? I was waiting for someone to chime in here but I thought the 12 bolts had different series'. Maybe that was just 10 bolts... I am never sure.

You're right... I was thinking 10-bolt. I'm a dork. Series carriers for the 12-bolt go like this:

2-series: up to 2.93
3-series: 3.08:1 to 3.73:1
4-series: 3.90:1 to 6.14:1

Keep in mind that many R&P manufacturers make thicker/thinner ring gears and/or spacers so that you can put different series gears on a carrier, but that comes with some minor reliability issues, and they are at a premium cost. I would consider just looking for a posi carrier with R&P at a swap meet.

If I were you, I might shoot for the 3.42 range depending on tire size and depending on how much you want to drive it daily. If its just a challenge car, go for more like 3.73. 3.90 or 4.10 would be too much. Let your converter choice make the torque happen. Final cam choice and tire size will determine how you fine tune that ratio.

Carrier Breaks: 2.76 / 3.07 / 3.90 / 4.10 from: http://www.ringpinion.com/DiffDetails.aspx?DiffID=191&SearchMode=Diff&TypeID=5

To the OP, It's often cheaper to find the whole rear with the right ratio than it is to buy new stuff for the same piece. Just get something around the same length and weld on your original brackets for "on the cheap" swaps.

I seem to recall there were different versions of a 12 bolt rear - Chevys got one version, and the differential used in Olds or Pontiacs was different. I don't remember details but it would be worth figuring out just what you have before spending any money. The Chevelle and Nova forums should have more information.

There was an "O" 12 bolt that was only one or two years I think and it has different guts and a cover than the regular one. This rear is one of the regular ones though so no worries there.

Now correct me if I am wrong but the 2.73 rear would be a better rear for mileage right? The place where it would hurt me is the all out acceleration though it would help on top speed. I may end up using it as is then look for new guts as I can run across them. I mean I got this complete rear for $100. At the very least it should hold up better than the even lousier stock gearing in my 7.5.

rebelgtp wrote: There was an "O" 12 bolt that was only one or two years I think and it has different guts and a cover than the regular one. This rear is one of the regular ones though so no worries there. Now correct me if I am wrong but the 2.73 rear would be a better rear for mileage right? The place where it would hurt me is the all out acceleration though it would help on top speed. I may end up using it as is then look for new guts as I can run across them. I mean I got this complete rear for $100. At the very least it should hold up better than the even lousier stock gearing in my 7.5.

I got my open 3.73 12 bolt for $20.

Holy crap how did you luck out on that? I have never seen a 12 that cheap.

rebelgtp wrote: Holy crap how did you luck out on that? I have never seen a 12 that cheap.

Actually another forum, 67-72chevytrucks.com. A member was selling it for $25 and only asked me for $20, even when the extra $5 was offered. That's not the lucky part, the lucky part is that the old one sold for $17.50 at the scrapyard. The new rear end was a net of $2.50!

Look around, open 12 bolts (and I know the truck ones are different) are around and plentiful. Cheaper than a new ring and pinion for sure.

Regardless of the gearing, isn't it one wheel peel? I picked up a posi Ford rear for $80. They're out there, is a GM imperative?

Dan

914Driver wrote: Regardless of the gearing, isn't it one wheel peel? I picked up a posi Ford rear for $80. They're out there, is a GM imperative? Dan

I'd say a Ford would be even better bet, but since he has GM brakes and wheels and he's entering the challenge, I was trying to stay cheap.

The one wheel peel thing could be addressed.

Well the pain in the butt part is the G Body cars use a fairly narrow rear compared to many of the cars you could get rears from. A ford 8.8 can be made to fit I know. Finding a 9 for challenge budget is damn near impossible.

rebelgtp wrote: Well the pain in the butt part is the G Body cars use a fairly narrow rear compared to many of the cars you could get rears from. A ford 8.8 can be made to fit I know. Finding a 9 for challenge budget is damn near impossible.

No need to get a 9 - an 8.8 would work fine. As far as width, keep in mind that you can get wheels with a different offset, and GM patterns are plentiful.

I'd stick with GM to make the U-joint and wheels (if you don't have to get new ones) and brakes work out with whatever you already have.

You can narrow an axle yourself. It can be done if you trust your welding skills. I do not trust mine. Not that much. You could also weld in a center section, again, if you trust your welds, if you're able to find that awesome posi 3.73 for $26 but it's too long.

In reply to tuna55:

Yeah I have several sets of wheels for the car as it is so I would rather not mess with getting new wheels at this point. An 8.8 might be an option in the future though.

As far as narrowing a rear myself I would not trust my welding skills considering they are basically non existent at this point. My life would be much easier if I could just find an inexpensive 8.5 10 bolt from a G body but those are so rare they go for a premium for custom builds and the like.