I just finished putting in a 3.42 posi in my '88 IROC (L03 TBI). It originally had the stock 2.73 open diff. This was the first time I ever touched a rear end. In fact, besides the fuel injection (new TBI unit and injectors and open element), and the shift kit I put in last year, this was only the third time I've ever turned a wrench on a car (well, I ported my new heads as well). The whole thing was done on jack stands in my driveway.

I got some really low mileage used gears and posi unit. The only specialized tools I had was a torque wrench, dial indicator with magnetic base and a two jaw puller. The gears and posi cost me about $150. The tools about $60. I also put on a Jeg's aluminum rear diff case/girdle and used a Ratech solid spacer instead of a crush sleeve. Including install kit, gear oil, posi additive and threadlocker, I got the whole thing done for about $450 or so.

And by the way, the gears and diff seem to be perfect ...rear end is completely silent. It's a very noticable difference in performance. Actually, I put on a poly trans mount and aluminum driveshaft at the same time. That stuff combined with the ported TBI and big injectors (90 lbs/hr), and the TransGo shift kit has really made a difference in the car - it's like a different animal. Much better acceleration and throttle response (I didn't change the AE parameters in the PROM when I switched to the big injectors so it's getting 60% more fuel when the peddle is suddenly depressed). The car idles nicely with the big injectors by the way ...and doesn't enter asynch mode as far as I can tell.

Anyway ...I am obviously a newbie amateur mechanic who just put gears and diff in my car (with solid pinion spacer) with the bare minimum of tools. Soo... if any newbies out there are considering doing the rear end in their car feel free to ask me any questions.

 

glad everything went well. not all are as lucky.

 

Haha, that sounds like experience speaking

Actually, I think I was really lucky in many ways. The diff and gears were low mileage (20000) and the bearings were left in place and in excellent shape. The stock GM pinion shim was on the pinion gear and it set up with a nice contact pattern (to my inexperienced eye anyway - and it turned out okay). So I didn't have to mess with setting pinion depth and removing and pressing bearings. Also, my stock GM differential shims gave me a good backlash (0.006-0.007) so I could have went with them (but I didn't - I screwed around a bit with shims to see how the contact pattern would change). I got the shims on the solid pinion spacer right on the third try. Seems like the gear install from heaven hey? Going with used GM parts made everything set up real easy. Of course, I took a chance on the parts, but they turned out to be in mint shape (not even enough use to detect a wear pattern on the gears - even the spider gears).

Actually, my pinion seal leaked because the one I got from Ratech didn't have any sealant ring on it and I was too stupid and lazy to put some gasket maker on (it was almost 12 at night and starting to rain). So I had to put the car up again and put on a new seal - no big deal.

Well, there's only about 25 miles on that rear end yet. It seems real nice so far, but it might suddenly blow up tomorrow

 

hey there. i'm lookin to put posi in my bird this month. i've already got 3.42 gears and i'm gonna stick w/ the cause of gas prices. i've been researching over the summer to figure out my specs and how the procedure is done. the only thing I seem to be confused about is how to use the dial indicator to make sure stuff lines up correctly. anything u got on that would help me out..... and is the posi you used a factory posi? gov-loc? thanks

-drew

 

Yeah, I used a factory gov-lock posi and factory gears from a 4th gen LT1 Z28. Went in real smooth. I got a good price on them so I went ahead. If they ever act up I'll get some aftermarket stuff - but I think they'll be fine.

The dial indicator is extremely simple to use. I got the $30 dial indicator and magnetic base from Summit (on an unrelated note, Summit's customer service has blown me away). You use the magnetic base to stick the differential case somewhere (where ever is convenient). Then you use the arms to position the dial indicator so it's plunger is up against one of the teeth of the ring gear. Then when you move the ring gear the dial indicator's plunger moves, and the needle on the indicator face tells you how far (in thousands of an inch) the ring gear moved. - The ring gear can move because it is not flush up against the pinion gear. The ring gear will move so far, hit the pinion gear and stop. So you can "wiggle" the ring gear back and forth until it brings up on the pinion gear on both sides. That amount of movement is your backlash - very simple procedure ...you'll figure it out in less than 10 secs

I aimed for a 0.006" backlash. Stock GM gears have to be pretty tight like that. You then use the contact pattern to tell you if you got your pinion depth right. I think most of these rear ends use the same size pinion shim so they swap fairly easily. In fact, even my stock differential shims gave me the correct backlash with the new diff and gears (though I changed the shims experimenting with backlash). Perhaps I was extremely lucky or perhaps all factory GM 7.625" rears use the same shims? Don't know but I'm not complaining

 

Good job on the install, Gov-lock were only used in trucks. You more than likely have an auburn cone type limited slip differential.
The Auburn type is much better suited to performance use.

I went from 2.73 to 3.73, talk about a wake up call, HELLO.
She is a totally different animal now.

Have the stock 2.73 ring and pinion for sale with only 30,000 miles on it. Email me directly at Jetmeck@aol.com. $ 20.00

Bill

 

I swapped a whole disc rear end (axle, 3.42 limited slip/posi) off a 1986 Iroc to my 1989 RS 305/L03 w/2.73 Open/Peg leg and the difference was phenomenal. You are right about the car being different. Only drawback was the speedometer was off which I never even got into since I sold the car to a buddy.