Does anyone produce a 12 bolt for a 73-77 GP?
 

Does anyone produce a 12 bolt GM rear end for a 73-77 Grand Prix, specifically a 1976. I've searches the internet and the only thing I have found is a Moser M9 through Spohn. I'd prefer to keep a GM type rear end under my car if possible.
Thanks in advance

Don't let the number of bolts fool you, the 8.5 is where I'd start

I would give curry or moser a call directly.

I can't see the the A and G body being so different that they couldn't build one for you.

I would think they would just need to adjust the tube length. You might have to add the spring perches yourself.

The '73-77 A-body r/e housings have a wider spread on their cast in upper control arm perches than the perches cast into housings used in all previous ('64-72) A-body rears, that would mean a different casting of the center housing by an aftermarket mfg like Moser. The typical 12 bolt doesn't have the pinion support of an 8.5 rear, so the extra 3/8" diam of a 12 bolt ring gear isn't the deciding factor in comparing r/e strength. Neither is the extra 2 axle splines, as OEM 12 bolt axles flare up at the beginning of the axle splines. unless the 12 bolt rear has 40+ year old GM HD service pkg axles or quality aftermarket 12 bolt axles, stock c-clip axles are a problem point.

Desire to stay GM rear, one can build up the '73+ 8.5 A-body housing with custom 30 spline bolt-in axles & an Eaton posi unit. I've built several '73 A9 housing A-body rears that way, as well a few with 33 spline spools & Strange axles for track use. What is the Purpose Build of the car, that should be the determining factor. Footbraked 11-12 second street car? Heavy low 11 sec car leaving with on trans brake?

Hello ol Pinion head, the plan for this car will mainly be to race on the weekends with some city/highway driving when I want to drive it around. The car will be heavy at about 4100 lbs or so with me in it. I'd like for it to ultimately run (with a 455) deep into the 12 second range if not the high 11 second range.

I know very little about upgrading rear ends so I don't know where to start and what would be good for the car and what would be a waste. I'd also prefer not to do any cutting on the original rear end. If I have to buy something from a different car to cut/weld up, I may go that route.

There was a '10-bolt' with an 8-7/8" ring gear that was used on '73-'77 A-body wagons. Our '76 Buick A-body wagon had one.

Factory gears for that ranged from 2.56 to 3.23.

I have this for sale... I bought this for my 77 GP in 1990. It is still brand new, 30 years old. Has never driven the car and only rolled maybe 100yds total. It is a Currie 9", was $4000 in 1990. It has 3.70 gears and a Ford Traction-Lock 4-pinion posi. I added Lincoln disc brakes after I bought it. This same rearend will still cost you $4000 today. Send me a pm if you are interested. (I don't know why the last pic uploaded upside down)

These did come in some GPS. My 75 has one from the factory. Aftermarket support is zero. Shame because they’re pretty strong. Mine has 31 spline axles but has C clips. Steepest gears are 3.42. There used to be a machine shop in Texas that made 3.55 gears for these to fit the most common 2.78-3.08 carrier. I upgraded mine from 2.56 open to 3.23 posi but searched for years to find good used gears and rebuilt carrier.

I gave up on trying to find anything like that for the reasons you stated. I can make my 10 bolt strong enough for what I plan on running at the track for half what it would cost for a fabricated Ford 9" from Moser. Putting a 9" in my Grand Prix is akin to blasphemy in my eyes. lol

The ironic thing is my car actually came with a posi rear end (1 of only 3 options the car has on it.), but the gear is like a 2.27:1. I'd use the existing carrier, but I don't trust the 44 year old 28 spline axles with a 455 in a 4000# car running deep into the 12's.

I have one of these 12 bolt ring gear, 10 bolt cover rears hanging in my shop if anyone is looking for one.

What gear ratio?