Found a posi out of an 86 t/a with disc brakes. I want to put it in my 91 with a non posi/rear drum set up. As far as brake cables/brake lines goes, will I be able to match up cables to this (91 is a 5 speed)? And for the prop valve, do I get one for a 91 assuming its different to the 86? Anything else thats a PITA worth mentioning, please feel free!!

 

Beware the pre-'89 iron delco calipers on disc rears;
they are leak-prone and the pistons can be damaged
if E-brake helix out of adjustment.When i swapped
disc rear into my car i used the master cylinder,
proportioning valve and e-brake cables from the donor.
I'm thinking '86 parts should fit '91(some of the brake
fitting threads changed~1984)
Also,if the posi rear from the '86 is a 9-bolt,check the oil
and if it is gray with iron filings or you find a bunch in
the rear,the posi is probably shot.

 

You'll also be downgrading your axles. Your 91 will have 28 spline axles. The 86 will only have 26 spline.

If you grab the entire diff to get the disk brakes and posi, you would need to buy a new 28 spline posi to use the larger axles from you 91 so in reality, all you're really doing is getting disk brakes at the expense of a new posi. Since your 91 doesn't have a posi, it would be a needed expense anyway to upgrade but using an older 26 spline posi is stepping backwards with an upgrade.

As mentioned above, the posi in the 86 is probably worn out so buying a new one for your 91 is probably a better option. An aftermarket disk brake swap would also be a better choice.

 

Are you able to identify it this rear has those calipers?
http://allentown.craigslist.org/pts/2968354496.html

 

the rear pictured is a 9-bolt borg warner with the iron
calipers-says it's a 3.08 ratio,not sure if 9-bolt offered
that ratio-2.77,3.27,3.45 is what i have heard of for
9-bolt ratios but others here may know more.
$250 is a good deal for that rear if it is good-the iron
calipers might not give you any trouble,but the
posibility for woe with them is there

 

Thank you for the info! I would love to be able to drop 2-3k on a brand new, but thats just not in the cards. Im having a hell of a time finding anything after 86 in the rear end department with a posi/disc. What year did they start the 28 spline axles? Are conversion kits insanely expensive? The rear in my car is EXTREMELY rusted, and its leaking out of one of the tubes (dont know the technical name, but halfway between the carrier and axle seal where it looks like a tube is coming out) and I dont know if its worth salvaging.

 

28 spline was in full production by 1990,with some late
'89s said to have 28 spline also-this only applies to the
10-bolt GM rears,9-bolt B/W is a completly different
design using (IIRC)27 spline axles.The 9-bolt B/W has
tapered roller bearings pressed onto the axles and the
axles are retained by a bolt on plate behind the hubs
much like some ford rears.The B/W differential has 4
spider pinions and the ring gear is 7.75" VS 7.625
used on '85 -up 10 bolt GM rears.
The bigggest weakness of the 9-bolt is the cone type
posi is an iron on iron design and is usually found
worn out with worn,gaulded cones and carrier halves.

 

IIRC, all 9-bolts were equipped with 28-spline axles (which were different lengths).

JamesC

 

Nothing from the 9 bolt is interchangeable with the 10 bolt. The only thing in common was external how it was attached to the car.

The only 28 spline axles that will fit into a third gen diff are from 90-92 or aftermarket. Fourth gen axles are longer.