fervernt

The only identifying stamp that I can find on my rear axle is Borg Warner and G7B7. There are no other stamps or tags on it anywhere. I was hoping someone would be able to identify what it is. I've had no luck with google, or anything else. I believe that the rear end was swapped, because my car has rear disc, and it came from factory(according to RPO) with drums. But I have no way to know because I can't identify the thing. it's pretty frustrating. also, I don't know if this is strange or not, but I've never seen it and I'm wondering if anyone can explain it. The calipers on the rear disc are strange. on the driver side, the caliper is towards the rear of the car where it sits on the wheel. however, on the passenger side, caliper is towards the front. any thoughts on that? badly done brake conversion, or is this normal?

sofakingdom

iTrader: (1)

Not much identification to do...

If this is in your Firebird, odds are about 99,999,999 to 1 that it's a 9-bolt.

Count the cover bolts... if it has 9, then guess what.

The calipers on opposite sides of the wheel is normal, for the crappy cast-iron caliper kind of brakes (Saginaw), that were used in these cars from 82-88; that allows the calipers to be the same on both sides instead of being mirror-images.

JamesC

iTrader: (5)

If that's the case in your 91, the rear end has been swapped in (a 91 with factory rear discs would have 11.7" rotors and alum PBR calipers which would be on the backside of the rotor). Since the 82-88 combination valve won't fit the thread pitch of your 91, the drum combo valve probably wasn't swapped out. I'd be somewhat surprised if the parking brake worked either. Finally, does the speedo read correctly?

JamesC

blue birdy 87

Take off diff cover it will have codes on the big gear think its called the spline that will say gear ratio. As for bolt count.... You count them, that simple

JamesC

iTrader: (5)

The numbers are stamped on the ring and pinion, for example (10-bolt):

41 : 15 = 2.73
41 : 14 = 2.93
40 : 13 = 3.08
42 : 13 = 3.23
41 : 12 = 3.42
41 : 11 = 3.73

JamesC

AlkyIROC

If it says Borg Warner on it and in your 91 then it's not original. It will be a 9 bolt diff. If could be a 2.77, 3.27 or 3.45 gears. An option set available back then was 3.70 but wasn't a production option. The 2.77 gears are on a 2 series carrier which is a door stop. 3.27 and deeper are on a 3 series carrier and have a lot more choices to pick from now.

Your original 91 10 bolt was probably a better differential. The 9 bolt is slightly stronger but replacement parts are expensive and hard to find. Sounds like someone didn't like the drum diff and felt a 9 bolt with disk brakes was a better option. They would have been better off to just upgrade the drums to disks on the 10 bolt diff. If they put a 9 bolt diff with 2.77 gears under the car then they really took a big step backwards.

The "one caliper on the front and one caliper on the rear" is normal.

sofakingdom

iTrader: (1)

Right; not original to your car. Original to 3rd gens but not to yours.

It's a 85-88 part, transplanted.

fervernt

JamesC: Believe it or not, the parking brake works great. I was surprised too when I found out that the disc brakes wasnt stock. Can't be easy to make parking brake work with a swap like that. professionally done maybe? also, speedo is around 10 mph faster than my actual speed from what I can tell.

AlkyIROC: So if it is a 2.77, how bad of a step back is it? also, if it's one of the better ones(3.27 or 3.45) then is it still a step back from what mine came with factory? I think mine was the most basic option available.

Wish I had known that those numbers were stamped on the gears like a week ago. I just changed the gasket back there because it was leaking lol.

fireturd350

iTrader: (8)

There is a great 9 bolt thread on this site. It has excellent info. If you can read and understand it you'll know more about 9 bolts than most mechanics.

2.77 is series 2 like stated on a 9 bolt. That's the carrier design (what the ring gear bolts to) and 3.27 is a series 3 which means it can use gear sets higher such as a 3.70.

You can net swaps for 200 to 300 on ebay or more from 9bolt.com. be prepared to spend another 200 to 700 for gear set and labor.

There is nothing wrong with having 2.77 for gas mileage but you don't want those for performance. Those can go 75 to 100 for gears and carrier on ebay. Not many people are wanting them.

In any case make sure its in good conditon if you are planning to swap. Mine had broken spider cross shafts so I sold my stuff and swapped in a free 10 bolt.

It's common for cones to wear and posi to start to slip and not perform correctly. The fix for that is also covered on this site too.

AlkyIROC

The 2.77 backwards step is because the only gear set that will fit onto that carrier (posi) is another 2.77 ratio. If you want any deeper ratio, you need to buy a new posi in a 3 series.

Kind of defeats the purpose of having a performance diff and only having highway gears in it.

fervernt

Well I pulled off the rear cover to replace the gasket on my differential, and I noticed numbers that looked to be hand written on the gears. looked like they said 7B 2558. I don't know if that means anything, but it's all I could find on there. but from my guesstimates and counting the teeth as best I could, I would say I do indeed have 2.77 gears in there. Fairly big disappointment.

sofakingdom

iTrader: (1)

The writing doesn't mean anything to me.

Finding the ratio is pretty much a no-brainer. Which qualifies me uniquely and totally to tell you how to do it, since I have no brain.

Count the teeth on the gears. Divide the # on the ring by the # on the pinion. 2.77 will be 36 and 13 (36 / 13 = 2.769230769230769230...)

fervernt

alright well I have a possibility for replacing that stupid 2.77 i've got on my car. bought a 92 v6 for the bumper and wheels, but I've been finding other good stuff on it too. If I did my research right, GU5 means that it's got a 3.23 on it. I'm thinking about doing the swap, but I'm wondering how difficult it would be to do. It's drum breaks, which is what my car had on it originally. has disc breaks now, and I'm wondering if it is possible to keep disc brakes on it with the swap. I don't mind the drums but I just put a lot of money into the rear discs and I'd hate to see it all wasted.(new calipers, pads, and rotors.) also wondering if its a good idea because the one I would be replacing it with has about 220000 miles on it. Any thoughts or suggestions are appreciated.