Can 4/5kq locking diff replace Torsen in 80q?

["Avram Dorfman" <dorfman@est.org>]

Does anyone out there know if it is possible (and relatively easy) to put
the locking diff from a 4000 quattro in as a replacement to the Torsen in an
80q?

For those who have followed my other emails, my brother and I are replacing
the synchros in his 80q. That is going well, but we also have a problem with
about 1/2 second of "slack" in the drivetrain on acceleration before a big
"clunk" and then finally actual acceleration.

One suggestion seems to be that the torsen is worn out. Car has 150k miles
on it. We have the torsen out, and there's 1-2cm of "play" in the rear drive
flange (measured at the outer edge of the flange circumference). Does anyone
know how much play there should be on a good torsen? I would imagine zero,
or like <1mm, but I'm just guessing.

Unfortunately, we have had ZERO luck finding a used Torsen. People are only
willing to sell us an entire used transmission for like $800 when we only
need the torsen. Nobody seems to be willing to take a torsen of an otherwise
bad tranny for us. New torsens cost (GULP) $2000!

So, we are wondering if we could just bolt a locking diff from a 4000q or
5000q onto this thing, housing and all. Do the housings match? Do the shafts
match? Of course we'd be on our own for figuring out how to *actuate* the
lock, but we're more worried about getting a good center diff in there. One
lister wrote me about a Torsen that behaved the way ours did, which actually
blew up on him & wrecked his tranny at the same time. So we're not excited
about putting the Torsen back in.

Of course we may have no more luck finding a 4/5kq diff than we did looking
for a torsen. My hope is that since an open diff doesn't rely on friction as
a major piece of it's functionality, that there wouldn't be any risk of a
used one being "worn out." Also, there seem to me more 4/5kqs in junkyards
around here (Eastern PA) than 80/90qs.

Any help would be appreciated!

-Avram