facelessnumber

Hey guys. I'm changing out my rearend Saturday, and I'm doing it at an apartment complex, so I need to be as quick as possible. If I can avoid it, I don't need to be going back and forth all day to the parts store, so I'm trying to get everything together at once.

I've got a '91 RS with drums. What's going into it came from a '95 Trans-Am. Disc brakes, posi, 3.42 ratio.

First, the brakes:

I'll be using the "rope clamp" method described in some other threads to handle the e-brake cables. I think I've got that figured out. The calipers are there and the rotors are good. I'll be doing brake pads just 'cause it's the Right Thing to Do. The rubber hose on one side is cut, so I'll replace that. The center hose, with the fitting where the two metal lines screw in, is cut. My first question: Do I need a new one, or will the one on my car fit? Second question: Proportioning valve. I can get the one that came from the '95. I can get the master cylinder too. Will it bolt on as-is, or will I need a flare tool? Should I get a thirdgen rear disc proportioning valve instead?

And the diff:

Somebody took the guts out thinking he could use it in an S10. He didn't. He put it back together but he says he's not sure if he got the shims right. Says they're in there, and I just need to see how they're supposed to go on and then I'm good. What the hell is he talking about? What should I check while I've got the cover off? And, this being a posi diff, what type of fluid would you guys recommend putting in there when I seal it back up?

Thanks!

Sonix

iTrader: (1)

posi from a '95 is probably a zexel torsen. I don't think you need to use posi additive with that diff style, for some reason.
If he's not sure about the shims, then you're screwed. Is it assembled, or just in the box, and you have to figure out which is left and which is right, and assemble the rear end? If you don't know left from right, you have to set it up as if you're putting in new gears - measure gear backlash, etc. There's going to be a left and a right shim, but if you don't know which is which, 99% of the time you'll screw it up. Murphys law right?

facelessnumber

Well I've done a little more reading about the shims, and asked some more questions.

Apparently the only thing that got removed was the carrier, and when it was put back the shims were pretty much just tossed inside the case. So, if it was right to begin with I can assume that if I put the shims back where the factory had them, the diff should be set up right. This is annoying, but I feel a little better. Can anyone point me to some useful information about the factory setup of this diff? A diagram, perhaps?

facelessnumber

So, there will only be two shims and I have a 50/50 chance of getting it right. Will it be obvious if I get them backwards?

Thanks for your response.

Anybody know somethin' about these brakes?

Pat Hall

iTrader: (31)

You can try the shims both ways, and whichever way gives you the least amount of slop between the ring and pinion gear is more than likely the correct way they go in. Technically, you're supposed to use a dial indicator and measure the backlash, but this method should get you by in a pinch. Make sure the chamferred side of the shims are facing the housing. The perfectly flat side of the shim is supposed to face the bearing. Your stock drum proportioning valve will work just fine with the aluminum rear calipers. I switched out the stock drum rear in my 89 RS to an aluminum disc rear, and the factory valve works just fine. Your stock rear line will bolt right up to the two lines on the 95 rear, they're the same thread size. As far as the posi unit goes, it should be an Auburn if it's the original one to the rearend. They didn't start using the Zexel-Torsen posi until 98.

facelessnumber

Thanks. Very helpful. So, I should just use a good gear oil, 75w90 with posi additive, right?

Using the stock drum proportioning valve, do you notice the rear brakes gripping any harder or softer than they should be? Of course my biggest fear in this would be to have the rear brakes lock up unexpectedly in a turn, but if that's not an issue then I'll probably leave the valve alone.

And, overall, since you switched to the discs and used the stock prop valve, how would you describe the difference in your braking from the drum setup? Much improved? Pedal seem firmer, softer, more or less travel? I assume of course you're stopping quicker too...

Sonix

iTrader: (1)

oh, learn something new every day.
ok, auburn, yep, you're right with the fluid.
if you've got some power going to this, you might want to put in some strength upgrades, namely a solid pinion spacer and girdle. But if not, I guess leave it.
Yea, perfectly flat side faces bearings, chamfered side is away from that. You can caliper the shims and see the thickness, but i'm not sure if you'll be able to tell by feel like pat mentioned. Give'r a shot I guess...

Pat Hall

iTrader: (31)

Oh you definitely don't have to worry about them locking up. In fact, even with the correct 4-wheel disc proportioning valve, GM didn't give the rear brakes enough line pressure due to safety concerns. The pedal in my car is nice and firm, and the height is just fine too. As far as stopping better, it definitely does, but that's probably more due to the fact that I put the 1LE front brakes on at the same time I swapped rears. The only way to get the ideal pressure bias between the front and rear brakes is to install a Wilwood adjustable proportioning valve in the rear line and completely eliminate the factory valve. I bought one for mine a while back, but haven't gotten around to installing it yet. Like I said, it'll work pretty good with the stock drum valve, but if you want it perfect, the Wilwood valve is the only way to go. There's a really good article in the tech articles section on here describing what you need to do to install the aftermarket valve. It involves using some adapters and cutting and reflaring some of the lines since the aftermarket valve is SAE threads and our lines are metric. You should be just fine in the meantime with your drum valve though. If you had the 88 and older cast iron calipers, you have to change the valve for sure, but the 89 and newer aluminum calipers work just fine with the drum valve.

facelessnumber

Excellent. Well I think I'm fully prepared then. I'll let you guys know how it goes.

Just one more question. I've got 16" single-line IROC wheels. '87 I think. I assume these will clear the brakes and fit just fine. Am I right?

Thanks again.

Pat Hall

iTrader: (31)

Those wheels will clear the brakes just fine. In fact, I even run my 15x7 RS rims with the PBR discs in the winter because I have snow tires on those rims. It's really close with 15" rims, but they actually clear, so you'll definitely have enough room with Iroc rims.

facelessnumber

Well, I did it yesterday. I'm pretty sure I've got it put together right. The wear pattern on the ring gear looks good. Although I didn't have any way to measure the backlash, I think it's fine. It's almost imperceptible but it's there. There's no noise from the rearend. The posi works great, (had fun testing that!) and I really don't mind the extra width. I was concerned that it would look hideous, but it's hard to notice and I'm sure it will improve cornering. The brakes are wonderful and I love the new gear ratio. While I was under there I replaced my swaybar endlinks, and put some air shocks on it. Now, I'm not a big fan of air shocks and the old-school jacked-up look, but I was concerned about the tires hitting the fender lip. I'm actually glad I did that, 'cause the old shocks were apparently worse than I thought. I put a conservative 60 pounds in them, and it's about half an inch higher in the back than in the front, but now I've got my peace of mind. I put a lot of weight in the rear today and hit some speedbumps pretty hard with no problems, so I think I'm set.

All this made a HUGE difference in the way the car feels. It takes off like a rocket, stops a lot better and it just rides a hell of a lot better overall. Feels like a new car.

Pat, Sonix - thanks a lot for all your help!

facelessnumber

Oh yeah, I'm gonna have to replace my speedometer gear(s). I'm running 245/50/16's. And assuming I've got the 15-tooth drive gear, I figure I'm going to need a 41-tooth driven gear. Sound about right?

Sonix

iTrader: (1)

Rock on! Glad it worked!

https://webdisk.ucalgary.ca/~jmknopp...eed%20calc.xls

Here's my excel spreadsheet to calculate the speedo gear you'll need to use. a thousand uses and counting for this sheet!
GM should have that gear, or else The transmission center should have it.

Yea, I went out of my way to get an inch pounds torque wrench with adapters to turn that massive 1 1/4" nut, to check my pinion spacer preload... ended up not using it, as long as there's no obvious drag, and there is no slop, you're set.

Pat Hall

iTrader: (31)

Congrats on the successful install! If you had 2.73 gears in your original rearend, then you've actually got the red colored 17 tooth gear on the output shaft of your trans. You'll need to get the gray colored 15 tooth gear, and for the cable gear, you need the yellow colored one. I can't remember exactly how many teeth it has, but it's the yellow one. I think it's either 41 or 42 teeth IIRC. These are the speedo gears I used when I switched from 2.73 to 3.42 in mine. I also run 245/50/16's. I was able to get them from my local Chevy dealership a few years ago. If you can't get them there, Bowtie Overdrives carries the color coded gears too. The only ratios that use the gray tranny gear are 3.42 and numerically higher. You have to remove the tailshaft housing to change the tranny gear, but it's not too hard to do, only 4 bolts to remove it. Sounds like once you get the speedo gears straightened out you'll be all set!

facelessnumber

Cool stuff, thanks. Guess I'm just going to have to open up the tailhousing and see what I've got. I think the original ratio on this car was 3.08, being a '91 L03, formerly manual transmission. But with all the stuff that's been done to this car, God only knows. The trans is definitely not original and not a totally stock build either. And I'm not even 100% sure anymore that my new rearend is a 3.42. It seems pretty low. The car it came from, ('96 TA, T5-6) is SUPPOSED to be 3.42, but I think I'll ride around with a GPS and look at the tach to verify it before I order my gears.

No big deal though. Minor inconvenience. The car hauls @$$ and I'm quite happy with it, whatever the axle ratio turns out to be.

When I get around to looking at the color of the speedometer gears and/or swapping them, do I need to be ready for a bunch of trans fluid to leak out?

Pat Hall

iTrader: (31)

Actually, they lose very little fluid when you remove the housing. Just have an oil change pan or other small catch can under there when you undo it, but it won't be a lot that comes out. You might want to pull the rearend cover off again when you go to check what ratio your ring and pinion are. The only sure-fire way to tell what ratio they are is to see what's stamped on the ring gear or the pinion gear. If you don't want to remove the cover again, you can jack up the rear and rotate one wheel a full 360 degrees while counting how many times the pinion yoke turns. If you have 3.42's it should rotate approximately 3 1/2 times for one full revolution of the tire.