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Changing synchros (an account from experience)

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.