im wondering what size rear i have. i know its a 3.08 drum but is it like a 7.25/7.50 or a 7.75?

and im looking into changing in a set of either 3.42s or 3.73s. would the size of the rear matter when i go to buy a set?

not really a tranny and rear person just would like to know so i can put a better set of gears in.

 

7 5/8" ring gear size, 82-02. Unless you know how to set up gears, that is a job that is well worth paying to have fun.

Be aware that there are 2 carriers. IIRC, 3.08 & down & 3.23 & up. So if you are wanting to go to deeper gears from a 3.08, it would require buying more than just the gears themselves.

 

if i buy a 4th gen rear would my third gen formula rims fit? other than brake lines and a proportioning valve would i need anything else to swap a disk rear in?

 

The gears are actually 1978-present.

They were originally 7.5", but sometime in the late 80s, they were increased slightly in OD, to 7-5/8" (7.625"). Doesn't matter, ALL the replacement ones are 7.625", and they fit all of the rears, even the ones that originally came with 7.5".

NOT 7.75". That is the 9-bolt size. You don't have that, you have a 10-bolt.

Yes your existing rims will "fit", but will look mortally STUPID. They will stick out 1-5/8" farther on each side, that being how much wider the 4th gen rear is. Since they are those little weenie 15" pizza-cutter things, they will look RIDICULOUS and POSEUR. Few things look more mismatched than little narrow wheels, that are so small you almost can't even buy tires for them any more, stuck out past the fenders. Looks somewhat OK from the side; but people behind you will be pointing and laughing at you (no, I'm sorry, I must be politically correct: laughing with you) because your car will look like it's riding on a cable spool. However, 4th gen rims will suck right up onto your car and look perfect, in fact will look just like they belong there; because they have — you guessed it — exactly 1-5/8" more backspacing. And, just as important, they will be 1" large diameter and 1" wider, which will make it ALOT easier to buy tires.

 

Wow. Kind words there sofakingdom. Ever try using a little tact & not being insulting? You sure did go off on the 15" wheel thing, but I see nothing to say what size he has. I see he says "Formula wheels" which are made in both 15" & 16" sizes, IIRC.

A 3rd gen rear offset 16" wheel fit great & doesn't stick out past the fenders. A 15" with the same offset & width would fit the same way. So the question of your wheels fitting is this....What wheels & tires do you have right now?

And plenty of people have swapped rears without swapping prop valves. Only reason to swap lines, would be if the threads were different, but I haven't heard about people having to do that, except for like the 82-84 cars, when a lot of things were still SAE threads & not metric threads.

Some people don't feel the difference in line pressure is worth doing the prop valve swap, some do. Personal preference I guess.

 

well that was a bit rude. and i like my rims thank you very much.

thank you blackenedbird for not being so insulting and rude. reason why i said prop valve was because somewhere i think i heard someone say that you need it for a disk rear to run the 2 extra lines. now that has me confused. is it just a hook up for to where the drum lines would have went?

without any type of insults or being rude..

 

you sure took your time writing all that. we could have done without all that unnecessary insulting. i love these rims!!!. its not hard to buy tires now. its 16 inch rim with goodyear gatorbacks.

 

Only the owner has to like it, nobody else.

245/50s on stock 16x8 rear wheels, on a 4th gen rear end under a 3rd gen will set the tires virtually flush with the fender lips, not out past them at all. Not a bad look & even if it had had 15x7s on it? That would only be 1/2" further in.

2 extra lines? You talking about the 4-channel ABS on the late LS1 rear ends? I've never looked at one of those myself, so I'm not sure how those are plumbed. But I would think there was still just 1 line to the back, there 2 separate ABS modules after the split. 1 for each wheel. Just guessing, since I've neve seen one.

I have a 95 rear end that is plumbed just like the 3rd gen rear ends. 1 line off the body, into a T-fitting on the axle.

 

I fail to see where anything I said was insulting?? All I tried to do was to tell it like it is. Sorry if the truth hurts your feelings.

OP: However all that may be, if you've got the 16" wheels, you're a bit luckier; they won't look quite as bad sticking out as the 15s would, but still kind of ... poseur. Cars I've seen with the 15" / 4th gen combo look OK from the side, as stated; the wheels appear to sort of fill the fenders, from the side. From the rear though, they look real spindly and out of place; you can see all kinds of space everywhere up under there. Makes the wheels look even narrower than they really are. Depending on exactly how your fenders are hung, the 16s might fit more or less flush, or might stick out very slightly at th etop; they stick out everywhere else BUT the top, that's for sure, although they're not as bad as the 15s, where the inner sidewall is so far out it's almost flush with the bottom of the fender which makes the car look from behind like the wheels are out in space and not connected to the car.

As stated, 4th gen rims just fit right up to a 4th gen rear in one of these cars, since their different backspacing exactly offsets the difference in the length of the rear. And since they're wider too, they look more like they fill up the ENTIRE space inside the fender well. Vette ones have the same general fitment.

They didn't start the different brake plumbing until the 4-channel ABS systems were added; sometime around 98 or so. Those have a discrete brake line all the way from the ABS unit to each wheel to allow individual control of each (hence the term "4-channel"), rather than one rear line shared by both rear wheels. The 3-channel ones have basically the same brake plumbing as older cars like ours.

 

i going to get a 94 rear with posi and disk and i was just making sure and ill be honest i have no clue what they were talking about with the lines because with brakes i just know the basic stuff like how to change and check the fluid.

245/50s are exactly what i have so i knew they were 16 inches.

so just to make sure. if i get the disk rear its basically just pull my old and install new. i know that it may seem crazy for me to want to do this when i barely know about it but thats the point i want to learn. so sorry if im making you have to repeat your self but im just checking before i pull the rear on my only car than have to go buy even more things to swap in my newer one.

 

well you sound a lot more experienced than i with the rears. could i run a 9 bolt from a third gen with 3.73s or would the 4th gen rear be a better investment?

my buddy had a 84 berlinetta that had a 305 carbed and a im assuming 700r4 but it had a 10 bolt posi disk rear with brake lines to each rear. the guy he bought it from didnt even mod it and he had it for about 4 years. even he bought it that way. so is that like a swap or a factory option?

 

Yes you can run a 9-bolt; bolts right into any of these cars. Not sure about the "investment" part. There was no 3.73 ratio available in that rear, stock, meaning you'd have to swap those into whatever rear you get, which would have to be a 3.27 or 3.45 to begin with for that to even be possible; and ANY parts for it are getting real hard to obtain. While it's somewhat "stronger", in terms of breakage, than the 10-bolt, it's probably not a good idea to swap to for a car that gets used much, because if you ever have any trouble, it could be near impossible to fix. The 4th gen 10-bolt is not really any stonger than what you've already got; its main advantages are a better posi (if you get a 98-up one), and the better brakes.

No the 84 didn't come with 4 discrete brake lines, i.e. 4-ch ABS, standard. It's extremely doubtful it had separate brake lines to each wheel; far more likely, it had one line from the prop valve to the rear that then went to a T in the center of the rear end and split from there to each wheel. While it might be possible to put 4-ch ABS in a carbed car, it seems like rather a serious engineering undertaking to fit an older car with all the sensors, wiring, pumps, lines, computers, and so on, that would be required. Not likely.

A 94 rear will have the same brake plumbing as a 3rd gen, with one line from front to rear and a T in the center.

 

Yes there was. The 9-bolt equal ratio to a 3.73 is a 3.77.

10-bolt 3.42 = 9-bolt 3.45
10-bolt 3.23 = 9-bolt 3.27
10-bolt 2.77 = 9-bolt 2.77

And getting 9-bolt parts only means you can't just walk into Joe-Blows Auto Parts store & buy them. 9-bolts are still in use to this day. But you can easily get them. Just gotta know where to look.

 

Since the bias is different from disc/drum to disc/standard disc, to disc/1LE, change the combination valve for the best braking performance. While the brakes will function without swapping, why bother if you're not getting the best out of the system?

The follwing 10-bolt FAQ contains info on master, combination valves, parking brake cables, and speedo gears along with brake info:

https://www.thirdgen.org/forums/tran...-10-bolts.html

From the above: The ten-bolt 7.5 (7 1/2) and 7.625 (7 5/8), introduced in 85, are virtually the same.

JamesC

 

Ummmm.... no.

The closest thing was a 3.70, which was only available over-the-counter, except that it might have come in the 85 L69 Firebirds.

For a great hint about how gear ratios work, remember that they are the ratio of 2 integers.

And as far as getting parts for those, they're getting harder to find all the time. Sure, you can find em today, if you know where to look; but what about tomorrow and the week after and in 2015? Not alot of sense in swapping in something that's just going to be a headache on down the road, when it's not "necessary" on any way.

 

85-90 9 bolt equipped f-bodies were available with either standard open differentials or limited slip versions as well as several different ratios including: 2.77, 3.08, 3.27 (G92 w/auto), 3.45 (G92 w/stick), and a few 3.70 ratio's came in some 85 & 86 Firebird's w/L69, stick & G92=Very Rare!

The above from the following info on the 9-bolt:

https://www.thirdgen.org/forums/faq-...now-about.html

JamesC

 

Call it a typo & not just typing 3.7. Just like where I said the 10-bolt was a 2.77. the 10-bolt equal is 2.73. But you didn't catch that typo...

2015 is only 4 years away & since brand new cars are being built with the gears? Then yeah....The parts be around in 2015, no problem. I'll take the stronger, bolt-in axle any day, over the weaker, c-clip axle. Sure it can be converted to a bolt-in axle, but that means more $ & you still only have 2 spider gears vs the 4 spider gears of the 9-bolt.

As JamesC shows...They did come bone stock in some 3rd gens. I knew that already too, but I had a 90 RS (that had it swapped in) with one years ago, before I even knew it was anything "special".

 

JamesC wrote the same thing about the 9-bolt 3.70 that I did. Not sure how there's any difference there?

Yes I'm well aware that 10-bolts came with 3.73s. I have one that came like that, sitting in my driveway. It's not particularly unusual. The 9-bolt 3.70 is FAR less common though.

The 9-bolt ratios usually have less pinion teeth than the nearest corresponding 10-bolt ones; e.g. the 10-bolt has 3.73 which is 11 & 41, the 9-bolt has 3.70 which is 10 & 37, although there is at least 1 AM supplier that has offered a 3.73 at some point in the past, which now may be discontinued like so many other 9-bolt parts; people complain about how hard it is even to find common maintenance parts like bearings and seals. Similarly the 10-bolt 3.23 is 13 & 42 where the 9-bolt 3.27 is 11 & 36, the 10-bolt 2.73 is 15 & 41 where the 9-bolt 2.77 is 13 & 36, etc.

Yes I'm well aware how far off 2015 is. I'm also well aware of how rapidly the availability of parts for the version of that rear used in our cars has dropped off in the last few years. Parts for our 9-bolts never were very common, and have only become less so over the time since it was most prevalent in the marketplace, which was around 1989. I have no reason to think that the process will slow down, let alone reverse itself.

I'm not sure why you're so dead set on trying to talk this guy into putting one of those into his car. Sure, it's a decent enough rear, as far as that goes; but jsut isn't wise to swap to at this point in history. Not alot of sense in convincing him to put something into his car that's likely to create trouble for him on down the road. Doesn't accrue any benefit to him, especially if he can only find one with the crappy Saginaw brakes on it. Please explain how going out of his way to spend money to install something with zero aftermarket support and an obvious path toward total obsolescence, is somehow a good idea for him (regardless of whether it's a good idea for you - 2 very different things).

 

I'm not. Just wondering why YOU are so dead set against it & keep bashing the 9-bolt? When the weak c-clip breaks, what happens to the axle? Where does is go? Out the side of the car. If a 9-bolt axle breaks, where does it go? Nowhere.

GM put the 9-bolt in the stronger cars, for a reason. GM wouldn't replace a 9-bolt with a 10-bolt for warranty coverage. But they used Dana 44 (IIRC the # correctly) to replace 9-bolts (not a 10-bolt).

I'm just about sharing facts & letting people make up their own mind, with their car & their $. Not telling people they are stupid for choosing one over the other. Why are you so dead set on telling him to go 10-bolt?

Now if you want about argue teeth & strength. The 11/41 10-bolt has 44/64 teeth.
The 10/37 9-bolt has 40/164 teeth.
Lets see....44/64 vs 40/164.

I didn't come in here to net-argue with someone. Just came in here to share info & not insult anyone. And there is aftermarket support. Not as much as the 10-bolts, but not zero as you say.

You are going to continue to try to argue & nit pick every little letter I type, so I'm done.

 

I'm telling him what I'm telling him, because he doesn't have a racing or other high-abuse situation, as far as I can see.

When he accidentally curbs his car one day in a snowstorm or something, where is he going to come up with a 9-bolt axle?

How many people have you seen with C-clip failure on the street, compared to bent axles? Which one is therefore the greater hazard? When you're in California, you build houses to withstand earthquakes, not hurricanes; when you're in Florida you worry about hurricanes, not earthquakes. Same basic deal here: what is REALLY the most likely thing to happen to somebody, specifically THIS GUY, and what therefore should they secure themselves against? I would support the idea that this fellow at this point in the power progression

needs to worry ALOT more about whether he can buy wear-out parts into the future, than whether C-clips are going to fail.

Why did GM discontinue the 9-bolt, and put 10-bolts in FAR higher-power cars than any TBI or TPI car ever thought about being? Why is it in the LT1 and LS1 cars if the 9-bolt is so much better? What do YOU know that GM doesn't?

I also didn't come in here to argue with someone giving bad advice. Unfortunately though, that seems to have happened to me. I come here to give sound advice: like, how to keep a car running well for the longest possible time for minimum cash outlay with minimum down-time and headaches. IMO putting a 9-bolt into a car that doesn't "need" one, is a step backwards by all of those criteria, no matter what about the C-clips. It's sending the guy down a path of frustration that will only increase as time goes on, a car that doesn't work while he hunts down scarce parts, and MASSIVE expense when something goes wrong; compared to something as common as dirt, plentiful spare and replacement parts everywhere both new and used for CHEEEP, and totally adequate for his application, as expressed in his description of his car.

Sorry if it makes too much sense.

 

I said I was done, but you dug yourself in deeper so.....

I know that GM discontinued 9-bolts because the 10-bolt was in-house vs importing the 9-bolt, therefore cheaper. The dollar line is the almighty decision maker for companies. Cheaper to replace a few 10-bolts than import a bunch of 9-bolts.

What doesn't make sense was your insuklting tone in your first posts. Give pros & cons, let people decide for themselves. The if the decide wrong (in YOUR eyes), then they have to live with the decision that they made.

I'm not saying go 9-bolt. I'm not saying go 10-bolt. I'm just saying what is what. No decide which is right for YOU. It is not my place to decide for anyone. Nor is it your place. THAT is what makes sense. What doesn't is deciding for people, what they should do.

You were off base in the very beginning with your 15" wheel comments, which he doesn't have. So you obviously make guesses in your decisions. Find out the facts of the person asking, before you suggest anything.
Quote:
305 tbi soon to be carbed
needs to worry ALOT more about whether he can buy wear-out parts into the future"

BTW....I know that carbs will NEVER completely go away. All of the racing community will see to that. Carbs will always remain in high demand.

 

wow i didnt know such a simple question would get this much response. s

so what were saying here is 9 bolts are "junk" and to just leave my lousy 10 bolt with 3.08 gears in? is it possible for me to buy gears for my rear?

carbs are the **** i think i love them. i dont see them going away, just finally getting cheaper!!

by the time july rolls around ill have about 5500$ to spend on my car. so i was planning on making it pretty nice. since it runs well now i was just going to upgrade suspension interior brakes and possibly if enough left over i want to swap in a better engine. now question is.

will my t5 with a rebuild hold up to a 350?

but back onto the rear thing. i want yous to tell me what i could get without going crazy and spending over 1500 on one. im open to all suggestions i still like the nostalgic 9 bolt idea but with all the criticism ill have to get a better rear and non-weenie rims LOL(no offense to you sofakingdom)

 

No, 9-bolts aren't "junk", by a long shot; a pretty good rear, as far as that goes. That's not the issue with them. Just, when something happens to one and you need a part RIGHT NOW, it ain't gonna happen and/or you will pay through the nose. And all of that will only get worse over the coming years.

Not a good idea to swap one in. Maintain, yes; swap in, not so much.

No offense taken. From anyone. Even though someone seemed to be trying.

IMO the 4th gen idea is much better. Get some rims that let you run modern tires, and it will be fine. You can get spacers for the front so you don't have to have mismatched wheels. About 1¾" - 2" is perfect, depending on what wheels you end up with.