My neighbor has a posi unit out of a thirdgen IROC I have an 87 lt with a one wheel wonder, he said he would give me the posi unit, my question is how hard of a swap would this be? I have re built just about everything on a car except for the rear end and I am a little leary about this, now he seems to think all we have to do is pull the carrier put my gears on the posi and slap it back together? would like a little advice about this before I move forward with it.

thanks ahead of time guys.

 

That's pretty much it, in the simplest case. Take your old carrier out, put the new one in using the shims that came from YOUR housing (NOT the ones that were in the housing the carrier came from), done. Check the backlash of course, and maybe have some replacement shims on hand if you break one or if the backlash needs re-set; but most often, it doesn't.

However there are a few potential issues you can run into.

The most obvious is if his carrier came from a 9-bolt, but your rear is a 10-bolt (or vice-versa). Internal parts from the 2 models are TOTALLY non-interchangeable.

The next most obvious is if the carrier is an Eaton Gov-Lock aka Grenade-Lock. You don't want that.

The next most obvious is the "series" difference. Low numerical ratio gear sets have a MUCH larger pinion gear, since the gear ratio is not only the ratio of the number of teeth, but also the ratio of the ring diameter to the pinion diameter. So, since th ering is always the same diameter, then as the ratio increases, the pinion must get smaller. The ring therefore must get thicker. At some point in the progression of ratios, it gets SO thick, that it's expensive to make; so at that point, they make a "break", and move the ring gear mounting flange on the carrier over toward the pinion, and start over with a thin ring. In the 10-bolt, this happens between 3.08 and 3.23 ("2 series" vs "3 series"). For that optimistic scenario above to work out, both rears must be the same series; 3.08 and lower, or 3.23 and higher. Clearly, you can take a 2 series carrier, and use existing 3 series gears on it, by adding a spacer; but since spacers aren't standardized, you might have to re-shim the carrier to get the gears exactly the right distance apart (backlash). It is not possible to put 2 series gears on a 3 series carrier, unless you know where to get a spacer that TAKES SPACE OUT instead of PUTTING SPACE IN. Maybe somebody else knows where to get one of those, but in THIS universe, I sure don't.

The last, and least obvious, problem, is the axle spline count. 88-back rears came with 26-spline axles (and the side gears inside the carrier that the axles fit into); 90-up use 28-spline. 89 could have either but they're usually the older one. Since it's so easy to swap rears (2 LCA bolts, 2 TA bolts, 4 drive shaft bolts, 2 parking brake cables, 2 shock nuts, 1 hydraulic line, sway bar hardware) and such a common item to tear up, the year of the sheet metal is no guarantee of the year of the rear. Check before buttoning it up.

 

Wow that was a mouthful! Thank you so much, I have seen the posi and it has gm stamped on it but I didnt see much more than that of course I really wasnt looking either so when he gets home from work hopefully I can get up there and take a look at it.

thanks agian