Can anyone tell me where to find the axle code for the above combination? Is it in the VIN code or the RPO list?

 

It would be on the RPO code. Located on the backside of the console glovebox.

 

Finding the RPO sheet in a 85 car is gonna be a crap shoot. I don't think they had started putting the SPID sheet on the glove box lid yet at that time. Sometimes you can find the car's factory build sheet under one of the seats, butt that's probably the best you can do in a car that old.

The ratio code is also stamped on the axle itself. 3 characters, starts with G. Like GT4 or GU5 or something of a similar structure.

That said, "codes" are kinda the hard way around. Better to just look at the parts.

Since you're gonna be catching up this very weekend on all the deferred maintenance that the car has accumulated over all these years, which will of course include changing the rear end fluid, you can just LOOK AT the gears while you're at it. Count the teeth: there will be something like 12 or 13 on the pinion, and 41 or thereabouts on the ring. The ratio is the one divided by the other. For example 42 & 13 would be 3.23 (42 ÷ 13 = 3.230769230769230769...), which is possibly exactly what you've got. And to avoid another post, make sure to LOOK AT the parts and see whether it's (or more accurately, may have been at one time) a posi, since that "code" (G80) isn't on the car anywhere, only on the build sheet.

 

I'm not sure if the axle is 7.5, 7.625, 8.2, or 8.5" (10 -Bolt) Is this the diameter of the pumpkin housing? or the diameter of the ring gears? I want to put in 4:10 or 4:11 ring and pinion set...THX...Joe

 

It's actually too cold right now in "Up-State" NY to do any work in the unheated garage, so I'm sourcing and shopping for parts right now..I've already purchased new flex front brake lines, calipers, pads, brake fluid, master cylinder cover, and gasket, and Spark plug wires...

 

85 Camaro would have come with the 7½" 10-bolt. Those numbers are the OD of the ring gear. 7.5" and 7.625" are the same thing; just, at some point in the mid 80s, they added 1/16" or so all the way around the edge, and widened the pinion teeth accordingly, without making any other changes. I don't even think any of the so-called 7½" gears you can buy nowadays are still the smaller size; AFAIK they're all 7-5/8" now. The 2 terms are used interchangeably.

The 8.2" 10-bolt was discontinued in about 77 or 78 so it's certainly not that.

The 8½" one was used in full-size cars and F bodies up to 81. None were ever used in 82-up F cars. So it's not that either.

85 - 89 TPI Firebirds came with the 9-bolt, as did many 87-89 TPI Camaros. That has a 7.75" ring. A few 90s got them butt only a VERY few. So you don't have that either, unless somebody has swapped one in, which is quite possible since they're completely interchangeable as far as fitting in the car.

If you REALLY have 3.23s, which is probably what the car would have come with, then you can put any higher ratio in there that you want. Be forewarned however, TPI is such a monstrous drag on high-RPM performance, that if you go too far, you'll end up with one of those deals where the car will take off like a jackrabbit and roast the tires if you're not careful, butt won't be able to pass a semi on a 2-lane road at 55mph unless you're going downhill. 3.73 is too much even, unless the motor has been at least somewhat uncorked; specifically with headers, which is about all you can easily do to a TPI 305 that actually helps it. Also, it's not likely that you have a posi; or that even if you do, it actually still does anything. It would have been the Auburn complete POS that doesn't last and can't be rebuilt. You'll want to replace that if you upgrade the gears. Problem is, there are few or none that will fit your 26-spline early POS axles; all the better units are made to go with the larger 28-spline ones from 89-up. You'll need to replace those as well. Which isn't a bad thing, really; stock axles are made of GARBAGE material, and aftermarket ones are ALL made of far better metal. Since in that design the axle itself IS the inner race for its bearings, if the bearings ever go bad, the axles are automatically destroyed too; and the axle is by far the softest and weeeeeeekest part of that whole system, so it usually goes first. In fact, if I were the betting kind (I'm not... I'm sometimes a university statistics professor) I'd almost be willing to bet money that yours are ruined there just like everybody else's whether they care to admit it or not. So, you'll need gears, carrier, axles, and overhaul kit, and I'd recommend a solid pinion spacer to replace the crush sleeve, a set of LCA relocation brackets, and a "girdle" type cover such as the T/A Performance one. And of course, the factory did a SUPER CRAPPY job of assembling the axle tubes to the housing, with 2 plug welds on each side; virtually ALL of them have failed in at least one of those places. If your rear leeeeeeks from around any of those then it's because the welds have broken and the tubes are now loose in the housing. I've had some that were so loose I could move the tube around in the pumpkin by hand. Problem with that then is, the pumpkin is cast iron, and the axles are steel, and to weld those 2 materials together, they need to be at least 750 or 800°F. As you can see that's a giant rabbit hole you're staring at going down into, and it makes absolutely ZERO sense to spend that kind of $$$$$ on a 88-back core either with drums or the s****y 82-88 cast-iron disc brakes. Best to get yourself a 89-92 one, and spend your upgrade money on that instead, and throw the 85 trash in the trash where it REALLY belongs.

 

Yes I found it on the bottom of the console lid. it shows G80...Positraction...THX, but what would designate the axle size? 7.5, 7.625, 8.5??

 

Wow, Ok thanks for all the great info. I found the GM RPO sticker and see a G80 code. According to crawlpedia, G80 is a positraction differential, but the code should also designate the size as well. It's not much help in identifying the size of the axle...

 

The axle size is designated by what year the car is and that it is 100% exactly as it was when it rolled off the GM assembly line. GM F-body cars never had a 10 bolt Saginaw rear end bigger than the 7.5"/7.625". At some point in time the 7.5" became a 7.625" (mid 80's). At some point in time the 26 spline 10 bolt became a 28 spline 10 bolt. Sofa thinks it was 1989 and I think it was 1990.

At this point in time 40 years later God only knows what rear end is in the car now and what parts are in it. That is unless you have owned this car since 1985 and have a perfect memory or detailed service records for your car.

Rear axel housings have an axel code stamped into them on the axel tube identifying what they are and 40 years later they can have anything in them by now.

 

it’s like you didn’t read sofas post !

 

I am in the same boat. Just purchased an 85 with 71,000 on her. Working on getting her road worthy as she has sat for 10 plus years. It has the GU6 axle code 3.42 and no posi. Would like to upgrade to a Posi rear. Going to pull the diff cover and see if in fact the original gears are still in there and change the fluid while I am at it. Then decide to look for a new complete rear differential assembly or replace the axles and carrier and such as @sofakingdom has stated above. I am open to opinions.

 

I already told you what size it is. It's the 7.5" / 7.625" 10-bolt. I also told you it's NOT either the 8.2" or the 8½". Which digit of any of those dimensions is not clear? I'll be happy to explain further, as necessary.

The problem with "codes" such as are found in gloveboxes and such is, NOBODY changes the "codes" in the glovebox when they change THE PARTS. For example, I have a 83 L69 Z28. Came with a 7½" 10-bolt just like yours did, except with 3.73 gears. It had drum brakes and no posi. Posi carriers in that time frame were SCARCE, and VERY few cars from those days had it as a result... in fact lots of cars with G80 on the build sheet DID NOT in fact have it when delivered, butt the buyer could have the stealership order the parts and bring the car back to them when the part materialized, as they still do for many other things such as optional wheels.

My 83 now has the rear from a 91 or so car, with an Eaton posi and 89-up PBR disc brakes. I don't have a build sheet or a SPID sheet, but I WILL bet you MONEY that if you came up with one for my car, it would STILL say that it has drum brakes and no posi. I did however leave the weenie stock gear ratio in it (3.73) so at least the GT4 code is still accurate. My 83 also has a T-56 from about a 96 or so Z28 or TransAm in it, butt the "codes" will STILL say T-5. What efffffing good are "codes" in such a case?

GET YOUR HEAD OUT OF THE "CODES" CLOUD. If you REALLY want to know what parts you have, LOOK AT THEM. They are what they are, whatever they are, no matter how strange, and no matter what the "codes" were, and even if they seem to be "features from the future".

As I told you, G80 is the code GM uses for "limited slip differential". (as opposed to "open", i.e. UNlimited slip, differential) GM has used that RPO code for decades in all its product lines. It by itself DOES NOT tell which model of rear you have. For example, my 04 Avalanche also has RPO codes G80 and GT4; GT4 was still the RPO code for 3.73 in 2004, butt in that vehicle (2004 8.6" truck 10-bolt) it's an Eaton Gov-Bomb STEAMING POS not an Auburn POS. If you find the gear ratio code which is also a code starting in G as I wrote you above, that will "decode", in a way, to the rear end model it came with, in that the "codes" in our cars for 10-bolt gear ratios are different from the "codes" for 9-bolt ratios. (as the ratios themselves mostly are) OTOH the G80 and GT4 in my truck, THE SAME "codes" my 83 Z28 has or might have had, will NOT tell you that they are 2 completely different rear ends: you just kinda have to know that a truck has a truck rear end, and a car has a car rear end. And on yet another hand, you may very well have a 9-bolt in your car (COUNT THE BOLTS to see), in which case the "codes" are ALTOGETHER USELESS, since THAT rear COULD NOT POSSIBLY have come in your car to begin with. Note also, that 9-bolts are almost all posi (not "all" though: I've seen a photo of one that wasn't, and it was enough years ago that I'm pretty sure it was before photos could easily be doctored that much) and therefore a car with one of those ALSO has the G80 code in yet this other model of rear. EFFFFF the "codes". LOOK AT THE PARTS.

Since the ONLY 2 original factory rears that can be installed into these cars are those 2 models, it's CERTAIN that you could ONLY have either the 7.5" / 7.625" 10-bolt or the 7.75" 9-bolt in it now, unless somebody replaced it with one of the few dozen Dana 44s that GM sold at stealerships over the counter in the late 80s, or has put an aftermarket GM 12-bolt or Frod 9" or Dana 60 or Frod 8.8" in it (those being the available aftermarket products for our cars), or done AHELLUVALOTTA fabrication. And since Camaro only came with the 10-bolt until 87, it's equally certain that that's what your car came with. (note carefully that I tiptoe around saying "what your car has")

"Codes" won't tell you diddly-squat if somebody has changed out the whole rear because they tore up the original, or changed parts like gears or whatever because ... humans, or ... who knows what. The "codes" ONLY tell you WHAT THE CAR CAME WITH, butt they DO NOT tell you WHAT'S IN IT NOW. Which one of those 2 things is more USEFUL to you???? FOCUS on "useful", as in, "what do I need to buy parts for", or "what should I be looking for to replace this POS I've got", or questions like that, instead of USELESS "codes". Buying parts for something your car came with butt that isn't there anymore is kinda STUUUUPID. If my 83 needs rear brakes, should I look at the "codes" and order shoes for drums because I'm too lazy to get out and LOOK AT THE PARTS???

Lay off the g-d "codes" and LOOK AT THE PARTS.

This stuff just ISN'T THAT HARD. You're trying to turn a CAR into rocket surgery or something. LOOK AT THE PARTS. They'll tell you everything you need to know.

 

 

Note that in those charts, the 7.625 refers to the 7½" 10-bolt (which had apparently got its little upgrade by then), and the 7.75 is the Borg-Warner 9-bolt. 2.73, 2.93, 3.08, 3.23, 3.42, and 3.73 are the common 10-bolt ratios in these cars; 2.77, 3.08, 3.27, 3.45, and 3.70 are the corresponding 9-bolt ones. AFAIK 3.70 gears for the 9-bolt are no longer available, butt 3.73 nowadays is; which is VERY popular (to the extent that ANYTHING about buying $$$parts$$$ for that rear is "popular") among people putting engines with some RPM capability into their cars, such as LS swaps.

 

If knew all the info you posted I wouldn't have posted the question...Thanks for all the great info you posted. Sorry to have irritated you with the codes, but as a novice on the axles, I started with the codes for info first off...

 

GM just should have stuck with the 12 bolt axles they used in the earlier Gen models. It would have prevented all this confusion...And Code B/S...

 

The car 12-bolt (which was not the same as the truck one) was discontinued around 72. A decade gone by the time these cars came out.

They could conceivably have stuck with the 8½" 10-bolt, butt the last year for those in cars was 81, in Camaro/Firebird. It was dropped out of all the other car platforms one by one during the late 70s update cycles starting in the 77 model year. F body was the last platform to get that particular update cycle, which of course happened for the 82 model year.

I suspect they went with the 7½" in every car platform after that (with RARE exceptions) for gas mileage reasons. None of the motors they were putting in cars really exceeded its limits anyway. All that machinery would have just been muda https://en.wikipedia.org/wiki/Muda_(Japanese_term).

I don't think people at GM were all that confused about it, themselves; which is all that matters to them. Not sure in any case what's confusing about any of it. It has all been explained to the fullest. All anyone has to do to avoid confusion is to decide to stop being confused, and instantly whatever seemed confusing before, will all become clear.

 

Retooling for the 10-bolt 7.5/7.625 8.2, and 8.5 etc. seem like a waste of time, money, and productivity. They had a proven 12-bolt car& truck axle and should have carried them over or implemented them into the newer car models, rather than manufacturing additional axles with different dimensions, number of splines-26, 28, 30 etc. Creating all the unnecessary confusion...

 

Not sure about this "confusion" thing you keep talking about. There's nothing "confusing" about any of it. No "confusion" exists except in the minds of those who choose to harbor it. It just Is What It Is.

They HAD TO improve vehicles' gas mileage at the various times they dropped the 12-bolt and introduced the 8½" and later the 7½" rears, and reducing the amount of sheer unnecessary machinery and weight is one of the best ways to do that. For example an Iron Puke 4-cyl probably couldn't even MOVE the cars like ours that they put them in if it had a 12-bolt in it, much less drag it up a hill. The 8.2" dates back to the 60s, before anybody gave a rat's hind quarters about gas mileage or even hardly knew what that was, and was no stronger than the 7½" (it used the same axle bearings for example), it just consumed more power unnecessarily than the newer rear. They designed and built rears of adequate dimensions to get the job done in their various applications as they saw "adequate" at those times and in their view. (which, granted, their idea of that at the time might not be the same as ours now) To use one GIGANTIC part in everything, whether it needed it or not, would have been just about as inappropriate as putting a 454 in every vehicle.

Also not sure about "proven"... all those rears are the same basic design, just, some of the parts are different sizes. There's nothing to "prove" about any of it.