I bought a rear end out of a fbody that is 3.42 (I counted teeth) but not knowing what year it came from how do I know anything about it? Is there any way to tell which year of fbody it came from? Is it 3 or 2 series? 28 spline axles or 26 or 30? Or do I just have to open it one more time to see? I'm asking cause it's time to get posi and I want $99 deal

 

The only way to be sure about the splines is to pop an axle and check.

If it's a 2-series carrier and you have the "special" 3-series gears for 2-series carrier, you can't use the $99 posi without replacing the gears; and if you have 26-spline axles, you can't use that posi without replacing the axles.

You can tell if they're the "special" gears by their thickness. The ring gear flange is moved over toward the (smaller) pinion by about .400" on a 3-series carrier as compared to a 2-series one (since a 2-series pinion is larger); so the "special" ring gear is about .400" thicker than a normal 3-series ring gear. A "special" 3.42 ring would be about 1" thich from the bottom of the valley of the teeth to the mounting surface, as compared to about 5/8" in a real 3-series gear.

If the rear has PBR brakes, odds are pretty good it's 90-up, which would be the 28-spline axles.

 

It's either 26 or 28 spline. Only 12 bolts used 30 spline axels stock.

There's probably no way to tell the year definitively. By running part numbers and stamping codes you may get a 3-4 year window, but that will take you more time than just pulling an axel and checking. GM wasn't very good about year specific casting and stamping numbers.

 

I've got drum brakes right now
I think it originally came from some V6 fbody
So if I wanna use this posi unit I'd have to buy new axles, right?
Anybody makes posi for 26 spline axles? How much is it? Maybe that would be cheaper than buying axles and posi?
Any other ideas for upgrading my rear end? I wanna go cheap cause I'm selling that car next year.

 

Several companies make a posi for the 26 spline rear. They start around $350. New axels will be about $250 for a 28 spline rear with drum brakes (they will have to be custom made, because this combination didn't exist from the factory). I got my axels from Moser and the quality is excellent. The 28 spline is a worthwhile conversion if you need the strength, since the axels are 10% stronger stock, and considerably moreso with aftermarket materials.

 

Insert loud obnoxous buzzer sound here....

This is just plain common ordinary wrong. There are probably at least a hundred thousand 6-cyl F-body cars from 90-92 that have that exact combination.

The only way to know for sure what axle spline you have is to take one out and look.

 

Well, that is what I was told when ordering axels, and I called about six different companies. No one makes an aftermarket axel for a drum rear with 28 splines. Moser was the only company that would make them. I had to take all of the measurements typically associated with having custom axels made, so even if it was a factory possibility in low performance models the aftermarket doesn't reproduce them.

That being said you can probably get a set of plain replacement axels for the '90-'92 car mentioned. Should work fine, but they won't be as strong as the Moser units I mentioned.

 

A good axle to get is from Superior. They are heat-treated after splining them, unlike "custom" axles; so they're a better part to begin with, in applications where a "custom" axle isn't necessary (i.e. where a factory configuration will work).

For future reference, there is no difference in the axles from disk to drum; except that in some of the older (80s) rears, the wheel flange on some of the drum ones was larger diameter than the disk ones, and wouldn't fit inside the disk rotor "hat". Disk axles will always fit drums. Later ones (AFAIK all 28-spline ones) are all the same, disk or drum.

 

Yeah, Superior offers nice products too. I run one of their 1-piece conversion axel kits in my CJ5 and have never had a problem with the rear in three years of hardcore flogging on the trail. One of the factory 2-piece axels broke shortly after I bought the Jeep.

Thanks for the info on the differences between the drum and disc axels. No one could ever give me a difinitive answer about that. I assumed that the bearing seat to backside of flange dimension may be different due to caliper mounting bracket clearance, because I've experienced problems in this area before when trying to swap drums for discs on other vehicles.

 

I couldn't say about the drum vs disk thing on other vehicles, since that varies widely; but these particular ones are easy. In fact, IIRC somebody on this board that has a machine shop actually turned their drum axles down to make them fit in disks.

 

I've heard of people doing that before. Not on 3rd gens specifically, but on other cars being converted to rear disks. Apparently it's a fairly common issue with such a swap.

When I converted my Blazer to rear disks it was a bit of a pain. I used a Warn full-floating conversion kit for the stock 8.5" 10 bolt, and bought the rear disc brackets. I used calipers off a Cadillac and rotors from a later model 1/2 ton truck that uses IRS. I had to have the centers of the rotors enlarged to fit over the hubs in the rear, I had to make spacers to allow the rotors to be properly centered on the lugs (holes in the rotors were too big), and then I had to machine a step in the caliper mount brackets to get the rotors to sit flush on the axel flange without the calipers binding. It was far from a bolt-on kit, as it was advertised. The improvement in stopping power in incredible though, so it was all worth it in the end.

 

I'll second that one. I have a 28 spline disc rear that had a bent axle. I pulled one from a 28 spline drum rear at the boneyard and it fits with the disc brakes no problem.

 

6 cylinder car? Why bring that up? Plenty of low optioned Third Gen V8 F-bodies got drums, and 28 spline axles. Discs were only standard on V8 f-bodies starting in 93.



Remove brakes, start car, set cruise, get file and go.

 

You can't use a 4th gen axel in a 3rd gen car. They are too long. The 4th gen cars have a wider axel housing to account for the change in wheel backspacing. If you put a 4th gen axel in a 3rd gen rear the wheels would stick out, but more importantly the bearing wouldn't be riding on the proper surface anymore.

You can swap a whole 4th gen rear under the car, but you need to run different wheels or the car will look silly, and the tires will probably rub.