I have an ‘88 5.7 TPI IROC, with a swapped T5. I’ve had a T56, freshly rebuilt I’ve hung onto for several years, that needs to be put in, because the T5 is failing.

I’ve waited due to my late wife’s illness, and because I was looking for a 3.73 diff. Took awhile, but I finally found a 3.73 posi disc 10-bolt from an ‘84 Firebird…but I have questions.

The seller told me the posi was shot, but for $300, I wasn’t worried too much about that. I’ve inspected the ring and pinion, and they look to be in great condition. The seller told me that new bearings and seals were installed about 3 years ago…so my questions:

What would you recommend for a replacement posi? The car will be almost exclusively street driven, with maybe an occasional trip to the strip.

Or, would it be smarter to simply put it in for now, and get the T56 swap out of the way, and replace the posi later?

What rubber line do I need to order to go between the diff, and the car? I’m confused on whether GM had gone to metric fittings by ‘88.

The diff has the ‘84 discs, I’ve read that there were several issues with those, and many have gone to the later discs. Is this true, and if so, what would you recommend replacing them with? I’m thinking it would be much easier to do a swap before I put the diff in the car.

Cost is something of an issue, but I want to do this right the first time.

 

This not really an answer to the rear end question but I say swap T56 when your ready and not let the rear end hold up the swap.

My camaro has an LT1/T56 and a 4.56 gear,last year the pinion let go and I bought a used rear end for cheap because I did not want to buy a new gear set and what ever else it was going to take to fix it at the time.

The used rear end had a 2.93 gear,I knew it would not be a good gear for the T56 but I went with it anyway.I was surprised that it drives really good but i do not use 6th unless I am on the interestate.

I plan on putting a 4.10 or 3.73 gear in when time permits but for now the 2.93 is working fine.

 

The 84 discs are the old cast-iron Delco-Moraine. Total crap.

84 was the changeover year from SAE to metric fittings. You'll just have to look and see what it has. Your 88 should be all-metric unless someone has been having their way with it.

I have the original stock gears in my car (3.73) with a T-56, and I can tell you, that's about the lowest ratio you'd want. 1st is like a T-5 taking off in 2nd, and 6th is useless below about 70 mph.

For a carrier I'd recommend a Tru-Trac, or a Torsen such as out of a 98 up F body; particularly if you can get the upgraded one from a SLP car. In either case, get a 28-spline one, and new aftermarket axles. The 84 rear has 26 spline garbage if still original.

I'd recommend either the 89-92 PBR brakes, or LT1 (93-97) which are very similar.

 

3.73 is what I was thinking, too, it just took a long time to find one. My son was trying to convince me that a 3.42 would be fine, since the 4th gens came with that, but I pointed out we live in the mountains, and 6th gear would be pretty much useless except on level or downhill runs.

4th gens are still fairly plentiful around here in junkyards, I’d assume it would be much easier to find LT1 brakes…thirdgens have become collectible, and have pretty much disappeared from the yards around East Tennessee.

I’m running Hawk’s 17x9 IROC wheels, so I’m assuming clearances shouldn’t be an issue.

Will the 4th gen emergency brake cable work, or will I need something else?

 

For a posi, I just purchased a Yukon Dura Grip (clutch style) from Quick Performance, for my '84 T/A. Will be installed shortly. My car has peg leg so time to update to a posi.
Since u are not racing, then a clutch style is fine for u. U do not need a locker.

 

Right: not a locker.

A worm-gear type.

In 2023, a clutch type is not the best choice. Familiar, yes; legacy, yes; "proven", yes; "best", not so much.

Kinda like, a flat-tappet cam, in that regard.

 

A set of gears is like $200. Chump change in a project of this scope.

Get whatever gear FITS you PERFECTLY. Life is too short for compromises when there's nothing on the line. EFFFF abuncha trying to find "used" ones. New ones are eeeezyer to set up anyway.

Can't speak to the parking brake cable. Not sure what an "emergency brake" is. No car I've ever had has been equipped with any such thing. Hold the car still when already sitting still, MAYBE, if the hill is moderate; "emergency", HELL no.

First car I ever had (60 Chevy), a brake line failed, coming down a hill at a traffic light. I rear-ended a Pinto. That Pinto rear-ended ANOTHER Pinto. And DAMMIT, neither one of em blew up. I was late coming back from lunch and had nothing to show for it. I was PI**ED. I paid for 2 Pintos, plus fixing my own car, for like 2 YEARS. My car's "emergency" brake made no noticeable difference whatsoever, in the moment, even though I locked it up as solid as it would go.