"85 IROC-Z28 Auto w/ 7.5 and or 7.625 axle 10 bolt, 26 or 28 splines?? I'd like to replace the center sec with a posi from Quick Performance, but don't know if it has 26 or 28 splines??? THX...Joe...

 

So eeeeeeeezzzy it hurts.

Pull one and count the splines.

If there are 26, you have ... you do the math. Same if you find 28.

Odds are somewhere in the high 99s that you have 26, since 28 didn't come out until several years later.

That said, only AN IDIOT would spend money on 26-spline ANYTHING. Best plan is, since stock axles SUCK anyway, is to buy a 28-spline carrier and a pair of axles to match.

 

I'm sure the VIN includes a letter or number indicating the axle code, but which one??? Thanks for the info...BTW did the '85 IROC's come with a posi rear end??? Or limited-slip??? THX...Joe I didn't want to go thru the hassle of pulling out an axle shaft to determine the # of splines...

 

Dude. Just remove the few bolts holding axle in, slide out, and count. You are working harder just trying to avoid the only sure way to know what is in there...

 

No axle code in the VIN.

Some came with "posi" (a slang abbreviation for "limited slip"), some did not. While you're looking at axle splines, also look at the carrier to see whether it has it; and count the gear teeth so you know what ratio it is, and spare us a post asking that too.

LOOK AT THE PARTS. It's the best way, and the only CERTAIN way, to know what you've got.

 

Isn't there a "C"-clip that hold the axle shafts in that has to be removed before the axles will slid out?? If so I'd have to remove the inspection cover and drain the fluid as well to pull the C-clip...

 

You're going to have to do it anyway to change the differential. It's all the same work either way, you're just being told to change your sequence and pull some parts first to verify ---> and then order new parts.

If you want it done then you gotta get to it. Nothing is gong to happen staring at it.

 

I agree, but before I get into it and disassemble it all, I want to order the right parts. It's also freezing here in NY so I want to avoid excess time in an unheated garage...

 

Again, it's all the same work either way, you're just being told to change your sequence and pull some parts first to verify and then order new parts.

If you want to cross your fingers and take your chances, then remember what Sofa said....

And then remember again what else Sofa said...

Blending your wants with his advice, I'd maybe say order the 28 spline axles and all the parts and then do the work in one shot. I've got a cold garage too, I hear your pain. But it's better than no garage! I put cardboard on the floor so I'm insulated from the cold concrete.

 

I'd like to add some 'been there done that', f-it's freezing.'

The first is that with the age of the car, who knows what may or may not have been swapped. Stock was 26-spline axles, but could have been swapped to a rear with 28-spine axles. Or even more interesting, a 9 bolt.

So diving in and taking a look isn't unrealistic.

If so inclined, on the passenger side, front/top of the axle tube is a stamped code. That code will define what the axle housing contained from the factory. May no longer be true, but it is as start...

One thing I've run into working on rears in a garage, and it is bad to find out the hard way. That the garage wall can get in the way of pulling an axle all of the way out of the diff. That will create some new 4-letter words.

If you were to do axles, or even not, the axle bearings may need to be replaced. And at a minimum the axle seals. Seals no matter, if the axles are moved, change the seals. Otherwise brake shoes and seals are next.

And NO RTV on the cover. Use a nice thick fiber gasket.

RBob.

 

Haha! Yep, that's when you learn the floor jack shuffle. Floor jack under the center section and tires about an inch or two off the ground. Push the handle left and right, and the car will walk with it a couple inches at a time. If you try to drag the car with the jack then the jack will just pull out from under the axle. But you can walk the car all the way across the garage wiggling the handle left and right.

 

So THAT explains the 6" round hole in the garage wall.........

 

Order 28 spline axles right before you start the work. Return if the car previously had 28s swapped in.

 

LOL! Been there done that with the floor jack shuffle. Just for a few inches luckily.

 

I've heard there's an easy way to determine if you have a posi by jacking up the axle and turning the driver side wheel (Left side) counter-clock wise, if the passenger side wheel (right side) wheel turns clock wise or the opposite direction, you have a posi. Is this true???THX...Joe...

 

I've seen this happen on an episode of Garage Squad. The axle shaft couldn't be removed because the I-beam post was in the way...LOL

 

I've seen this happen on an episode of Garage Squad. The axle shaft couldn't be removed because the I-beam post was in the way...LOL

 

The opposite, place transmission in neutral and rotate either tire in either direction. If the other side rotates in the same direction then there is a posi installed.

RBob.

 

Not exactly.

Put the transmission in Park if an auto, or high gear if a stick shift. Then try to rotate a wheel. If the rear is yerbasic "open", with no clutches or other similar mechanism, the other wheel will rotate the reverse direction, with almost no effort. (like, 5 ft-lbs or something) A "limited slip" aka "posi" (short for "PosiTraction", a GM tradename from the 60s for the Eaton unit they used in certain cars, which was of course short for "positive traction") will be VERY hard to turn under these conditions: factory spec for the required effort is something around 60 ft-lbs for most units, more or less.

But of course that kind of a test assumes that any installed "posi" is actually WORKING, which in these cars, is FAR FROM a given. The rear may have been equipped with a "posi" originally, but it's so worn out by now that it's totally without effect.

If the trans is in neutral, the rear will do whatever it finds easiest. Both wheels will turn the same way if that's easier than turning the drive shaft even if it's an "open" (non-"posi"), for example. Which renders the "test" WORTHLESS. Even more worthless than usual. OTOH if the drive shaft is locked, spinning one wheel by hand MUST force the other to spin the opposite way; and a "posi" will attempt to resist this, by way of its clutches. IF it's working, that is.

Best way to figure out what parts you have, is, has always been, and will always be, to LOOK AT THEM.

Meanwhile, since 26-spline axles are GARBAGE and stock axles of ANY spline count are also GARBAGE, the best plan for an upgrade is, buy a quality pair of 28-spline axles, and the 28-spline carrier of your choice; and new gears in the ratio you want. That way you don't even have to THIMK TWICE about what's in there now or what it came with.

 

Why no RTV on the diff cover? I've almost always used RTV on my covers and never had a leak. At the same time I don't know how many rear ends I've taken the covers off of that had leaky gaskets, and old fiber gaskets are a real pain to get off when they get fused to the parts over time because no one bothered to use a gasket sealer that allows for easy disassembly (like #2 sealant). And I don't understand what you mean by "nice thick fiber gasket": unless you make your own gasket out thick gasket material you're stuck with what Fel-pro or whatever brand gasket you have is made out of for your car.

The only drawback to using RTV is when you have to take things apart again because it's a real job to get it all removed once it has cured because it sticks tenaciously to (degreased) metal, but it's a lot easier to do this than try to take off a hard old stuck-on paper or fiber gasket. There are people who will tell you not to use RTV because globs of it can break off and get into your internal parts: IMO you'd have to be putting it on way too thick for this to happen - I've never seen this.

I think the main problem with RTV is people don't follow the instructions when using it and then they blame it on the RTV when they get leaks. In general you want to put it on one surface with a thin bead surround all bolt holes, put the cover on, put the bolts in only finger tight, let it sit for an hour, then torque the bolts down. Then you want to give it 24 hours or more to cure before putting your oil in. My guess is most people who have trouble with RTV just put everything together and tighten down the bolts and fill the oil right away after putting it on, then they wonder why they are getting leaks from what they just tried to seal.

 

There's a blank tag on the top of the axle, no numbers or codes. There are raised letters and numbers on the bottom of the pumpkin: GM, plus a few numbers that's all I could find. According to the 3rd Gen website, GM increased the ring gear from 7.5 to 7.625" in '85.

 

The "raised letters and numbers" are the casting number of the pumpkin casting. Which is, that cast piece at the center of the axle assy. That casting is unique to our cars... ALL of them, 82-92, and as far forward into the 4th gens as the introduction of ABS. (all w 10-bolt, that is) Then, they plug-weld 2 tubes into it, weld the end pieces on those (they vary according to drum or disc brakes), and load the result up with whatever carrier (open or posi, 2 or 3 series, etc.), whatever gears, and whatever axles. All without disturbing or changing that casting number.

That may be accurate, I can't vouch for it, exactly. It's at least close though. Needless to say, whenever they did that, they made the same change to S trucks, A B & G bodies, Astro vans, etc.; all the vehicles that used this model of rear at the time. The slightly larger gears fit all the earlier parts, and I don't think it's even possible to buy the smaller size gears anymore.

What did you see inside it when you popped the cover off? Whatever you see, is what's there.

 

Do not use synthetic lubricant in axles using RTV sealant. Synthetic lubricant will chemically attack RTV sealant.
You must use this:

And if using a gasket, wherever possible, glue the gasket to the REMOVABLE part.