repost: Changing synchros (an account from experience)

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

(Sorry for the Rich Text gobbledygook in that last one - here's a clean
post)

I can help you. My brother and I just replaced the synchros in his '88 80q,
about 2 months ago. It went off mostly without a hitch, largely because we
were able to avoid most of the aspects of the change that need special
tools, or are simply a real pain.

We only did 1st, 2nd, and reverse.

First of all, you must have the Bentley manual. All I can do is help you
interpret what it says (which takes som effort).

Second, here's a problem that we ran into that the manual doesn't mention:
Audi change the 3rd/4th sychro rings, springs, hub and sleeve, in around
1990, and they stopped making the synchros that go on the old style
hub/sleeve. So, if you have an old style 3/4 hub & sleeve, you'll need to
buy new ones. They were expensive, either $150 each, or $150 for the set,
i'm not sure. The rings will be approx $45 each. I can't tell you how to
determine exaclty which you have. We didn't need to change 3/4 synchros, so
we avoided this whole problem, for reasons I'll get to in a minute...

On one hand, people will tell you that if you do this operation, you'd be
silly not to change them all, or at least change 1st & 2nd while you're at
it, because they're the ones that are most likely to wear out. However, with
this tranmission, the 3rd & 4th synchros are on a different shaft than the
1st/2nd and 5th/rev synchros. 3/4 are on the input shaft, and 1/2&5/r are on
the drive shaft. If this confuses you, read on... Suffice it to say, the
hard part (i.e. need special tools) of rebuilding a tranny is the presses
you need to pull the gears off the shafts, and to press them back on. We
just took the whole internal drive shaft, bearings, gears and all, to a
tranmissions shop. We handed them the shaft, the new synchro rings, and the
Bentley manual, and gave them some specific pointers on details particular
to this transmission that they had to do a certain way. The pulled it apart,
put the new rings on, and put it back together for $90. I believe that was
1.5 hours labor. It probably would have cost the same amount again to get
them to do the input shaft with 3/4 on it, so we just blew 3/4 off entirely.

Ok, know for how a tranny works. For those of you who don't know, every
"gear" (e.g. 1st) is actually 2 gears, one that gets turned by the engine,
and one that turns the driveshaft. Furthermore, all 12 gears (1-5 + r) are
in *constant* contact (i.e. mesh). When you shift gears, you might visualize
toothed rings pulling apart and going together (I did). Not so. The way it
works is that of the 2 gears in each pair, one is fixed to the shaft that
it's on, and the other spins freely on the shaft that it's on. When you
shift into say 2nd, the 2nd gear synchro sleeve locks the spinning 2nd gear
onto the shaft it's spinning on. The synchro sleeve has teeth that meet up
with teeth inside 2nd gear. The syncho ring & sleeve slides back & forth on
a hub, which also has teeth. 2nd's teeth meet the sleeve's teeth, which are
already on the hub's teeth. The hub is fixed to the shaft, and presto,
you're in 2nd gear. The acutal synchro ring just sits in the middle of this
whole process, and rubs up against the inside of 2nd harder and harder while
you're shifting into 2nd, until the freewheeling 2nd gear is going the same
speed as the synchro ring.

Somehow (I don't exactly understand how), the synchro ring is shaped such
that it's very hard to make it spin & slide at the same time. This is why
you can't shift into gear when the gear isn't synchronized. I think it's
just that there's a lot more friction for you to overcome. Once the gear is
synchronized, you just have to overcome the sliding. This is just a guess,
it's hard to see inside these things when they're on the shaft. When they're
off, there's nothing to look at. Just rings & teeth. There may be more to it
than this at the physics level.

So, what it boils down to, is the fixed gears for 1/2, 5/r are on the input
shaft (the one that goes into the clutch), and the free gears for 3/4 are
between them. Conversely, the free gears for 1/2, 5/r are on the drive shaft
(the one that goes to the front & rear differentials), with the fixed gears
for 3/4 in between them. Also, for those who wondered, the synchros are in
pairs. One sleeve sits between 1st and 2nd, actuates the syncrhos for both,
depending on which direction you push it. Likewise for 3/4, and 5/r. And
yes, there's an extra gear for r. It's actually not on any shaft at all -
it's betwen the input and output shafts, mounted on the back of the housing.
It links up between the fixed r gear on the in put shaft and the free r gear
on the driveshaft only when you actually put all the shafts back into the
housing. This is how the tranny manages to make the drive shaft go the
opposite direction when you use *that* gear.

There's a lot more I could tell you, but it's better if you just ask
questions. Getting the trans out of the car took us 7 hours. We could have
done it in 6, but we were moving pretty slowly b/c it was hard to understand
the manual. Getting it back in took 4 hours, not including the time we
wasted trying to bleed the clutch not knowing that it's supposed to be
pressure bled. You don't have to bleed the clutch at all to do this job, but
my brother *wanted* to.

Incidentally, I'm in the Air Force Reserves, so we had the benefit of the
Auto Hobby shop on base, which had real hydraulic lifts, and a transmission
lift for carefully pulling the trans out instead of letting it drop to the
floor. A transmission lift is like a floor jack with a big pole on it going
straight up, and a table at the end of the pole. It's good for getting the
trans at just the right angle to pull it out. This took a lot of fiddling,
and I don't know how we would have done it otherwise. You'll need a
makeshift trans lift if you don't have a real one.

Here's a tip - if you get to steps in the manual that seem irrelevant, or
seem like it would be easier to do them later, you're wrong. Trust me.

-Avram