My son has a '91 RS that we have been working on. I bought a used disc/posi unit last year and put it in. I was told it was a 4th gen rear end and it would work but the width is slightly greater. I didn't measure the width between the one taken out and the one put in to make sure this was a true statement, but it did fit. It had developed a whine so I had my mechanic buddy tear it down to check it out. He said the carrier bearing was worn some and was likely causing the whine. He also said the spider gears have some chipping on the edges and they should be replaced while he's in there since we plan to put a new power plant in it in the near future. My son is in the Air Force and attending tech. school in Texas right now so I'm trying to take of it while he's away. Can anyone explain the difference between the two generations? My mechanic said it was a 7.5" rear end. Do I need a carrier for a specific generation or are they interchangeable?

technically its a 7.625 ring gear, but will interchange with a 7.5.

if it is indeed a 4th gen rear, its a 28 spline axle, and you need a carrier for 28spline axles. Everything else is interchangeable.

There's a sticky post at the top of the forum, documenting (FAQ) the 10-bolt rear differences as well as many other things.

There was no strength difference between the 91's diff and the fourth gen diff. The only thing you gained from doing the swap was disk brakes and a posi. Those could easily have been simply swapped to the 91's diff. The 91 also uses 28 splines axles.

the fourth gen is 3 inches wider

Hey,

Eventually I want to go with disc brakes on the rear so I could change the axels and the brakets over to mine and it will work fine?

Tony.

Hey guys I jst went and bought me a fourth gen rearend with an auburn posi and 4:11 gears, but the bearings are bad, could I jst put all the stuff from the fourth gen into a third gen rear I have? I need some help here.

Yes all the "stuff" will go right in, except the axles which are about 1-5/8" too long.

You will need axles like these http://www.summitracing.com/parts/SAG-EV10-4/ if your rear is a 89 or older. You could also use Moser or other "custom" axles; but these are better for a "stock" application, because "custom" ones by definition have to be left soft enough to cut spines into after heat treating, whereas these are rolled splines created before heat-traeting. Which is one reason why "custom" axles almost invariably HAVE TO be bigger and have more splines than production ones, all else being equal. Best to leave "custom" axles to "custom" applications.

Be prepared to replace the Auburn soon. I will NEVER use another of their products again if I can help it, due to their lightweight construction and ZERO cusotmer service.