79 firebird ring gear

I have a 2pj g3 rear end 2.41 ratio would like to up to a 3.42 not sure if this is a 7.5 of an 8.5 ring gear. Anyone know where I can look on the web for the info. I can find the number, that is where I got the ratio.  I can't find the ring gear size.  

Your 79 Firebird uses an 8.5" ring gear. 82 and later birds use the 7.5". When changing from 2.42,2.56, or 2.73 gears to 3.08 thru 3.90 gears you will have to change the carrier (whether posi or open, it doesn't matter). The reason is that the pinion gear in the 2-series gears is a larger diameter than the 3-series which moves the ring gear farther away from the centre line of the pinion, with the 8.5" rears, GM made up for the difference in the thickness of the carrier flange. If you try to use a 2-series carrier with 3-series gears, the ring and pinion will barely mesh, if at all. The later 7.5" diffs made up the difference in the thickness of the ring gear and all use the same carriers.
When shopping for used parts, look for late 70's- 81 Z28's with 350's and automatics or late 400/4sd TA's of the same vintage, you can often find a complete posi (saf-t-track) rear end to bolt-in rather than buying gears and posi separately and paying to have it set-up (it's not a job for at home unless you know what you are doing).  
A little help... 'cause we don't all have to learn the hard way!

Thanks, not real good news.  I guess I have some more junk yarding to do.  I have the 2pj rear and a set of 7.5 motive gear gears. gm7.5-342. I don't have much money in the rear end thanks to the 69 GTO wheels that it was sporting when I got it.  So if I find a 3-series carrier for a 3.08 thru 3.90 I can use the 3.42 ring and pinion if I need it?

Neil  

According to your last post, you have a set of 3.42 for a 7.5" these will not work in the 8.5" regardless of carrier.  
A little help... 'cause we don't all have to learn the hard way!

Ding, Ding I get it.  I am going to have to swap the ring gears for 8.5.  Find a carrier for a 3.something with 8.5 ring size that has 28 splines. I guess the 2.41 carrier is not able to handle the 3.42 ring and pinion.  Will that  have to come from a large V-8 model firebird up about 1981 or so or is the spline count on the 3-gen post 82 model 28 and will that carrier work?

Thanks for the education.
Neil  

Did some more reading. Looks like the 7.5 came out in 82.  Are there any GM or Chevy trucks that have a differential that will work?

Neil  

The 8.5 came in firebirds, camaros, novas (& clones) from 70-81. Also full size from 77-90something, not sure about trucks but I think some Chevy C1500, GMC C10 full size trucks had them. The S-10/15 were 7.5", as were all A/G bodies from 78-87 (except Buick GN/T-types and some Hurst Olds and 442, some had 8.5")  
A little help... 'cause we don't all have to learn the hard way!

That gives me plenty to work from.  I found several on e-bay cheap.  The vendor is swapping the ring and pinion set and I have some time to shop for the carrier.

Thanks again you are the rear end Yoda.
Neil  

I'd put some C-clip eliminators on that 10-bolt. I found out the hard way that the C-clip retainers are the weak link on the 8.5" 10-bolt. (Not fun!)

Geno