Hello, Everyone:
I just completed a rebuild of my K75S transmission and thought I'd share some highlights with the group. I have a 1994 K75S w/ABS at 70K miles. I had 3 reasons to rebuild it:
1) The shift lever had some play in it that had been getting worse.
2) There was a lot of loud clattering noise coming from the transmission when idling in neutral with the clutch lever released. As soon as I pulled in on the clutch lever, the clattering noises disappeared.
3) It was becoming difficult to change gears, even with a properly adjusted clutch.
All of these symptoms told me the transmission and clutch were coming off. I bought this bike last year from a guy I didn't know, so I had no idea of the actual service history and had no idea what to expect. I've been restoring the bike over the last 6 months (front to back), so most of the bike is in good shape, but this was the first deep fix I've had to do so far.
What I found:
1) The transmission input shaft splines were dry and worn to the point I had to replace the input shaft. This was USD345, which wasn't cheap! At least BMW still had some available. The clutch friction disc splines were worn as well, so I had to replace the clutch disc. I didn't want to risk wearing the new input shaft splines. The clutch disc looked pretty new and measured at 5,44mm, which is well within limits and was unfortunate I had to replace it so soon.
2) There was a significant amount of play in the input shaft. I could move it back and forth, in and out of the transmission almost 2,5mm! This was no doubt the source of the clattering noise.
3) After completely disassembling the transmission, I found that the front input shaft tapered bearing and race were heavily worn. Major pitting and scoring.
4) I found that the gear shift play was due to the conical retaining bolt (aka "grub nut") being very loose. It was starting to unscrew itself and, I'm assuming, was on the way to rendering the shift mechanism unusable. (This would have been a problem on a long trip!)
5) The gear sets were in decent shape. There was some light wear on the gear teeth, but not a problem. The driving dogs showed slight wear, but were still sharp and angular and also not a problem. The shift fork shafts were in great shape: not bent and no visible wear. No scoring in the shift drum channels. None of the bushings showed any noticeable play.
What I did:
1) Disassembled all components, including the input and output shafts (the intermediate shaft is not meant to be disassembled), shift mechanisms, etc. Everything was checked for bends, scoring, or any unusual wear patterns. Everything was oiled and reassembled to spec.
2) I changed all the ball bearings and tapered roller bearings and races.
3) I changed all the oil seals on the input shaft (both inner and outer), the output shaft, the shift drum/gear position indicator shaft (plus a new gasket behind the gear position switch), and the shift shaft.
4) I had my local dealership measure and shim the shafts. This was a good investment because new shims are about USD10 per shim. They swapped my shims for the correct shims from their service stockpile and were able to do a friction measurement on the pre-load of the input shaft tapered bearings. Also, they shimmed the end play on the intermediate shaft and output shaft. After all the money I put into this rebuild, I want the bearings to last a long time, so this was the best way to ensure the clearances were correct and that the input shaft bearing pre-load was spot on.
5) I put some Lock-tite on the "grub nut" and torqued it down.
6) Installed new clutch and oil o-ring on the engine output shaft.
7) Lubed the input shaft splines with Unirex S2.
Lubed the output shaft, drive shaft and final drive splines with Honda Moly 60. These splines were in excellent condition, which was very good news, indeed.
9) Cleaned and re-lubed the clutch actuator arm and replaced the clutch cable (my rubber gaitors and grommets were torn and the cable had bends in it)
One other thing: on re-assembly, I noticed the alternator clutch housing had broken fins. I replaced this housing with a new one and also got new rubber dampers. I never had a charging system problem, but this was obviously not right.
What I learned:
1) The input shaft bearing races were not easy to remove. I ended up having the dealership remove them with a special Kukko puller. There just wasn't enough of a lip exposed for me to get them out without risking damage to the transmission housing and cover, so instead of paying USD350 for a puller, I paid USD37 to the dealer to extract both races. I was able to install the new races with my shop press and some sockets on top of the old races. They went in very smoothly.
2) Rebuilding your own transmission can cost a lot, but I like this approach (to a point) better than buying a "good used transmission" off of eBay or Craigslist. The reason is, you never know what you're getting with a used transmission and may end up either buying more than one, or perhaps end up rebuilding it later. At least you'd have a lot of parts to pull from!
3) I have spent many hours reading forum posts on what spline lubes to use on which splines. There are a lot of opinions out there (many of them are dated before 2006), but I chose to use Esso Unirex S 2 (latest recommendation of BMW as of 2011) on the input shaft/clutch friction disc, and Honda Moly 60 on the output shaft, drive shaft and final drive splines. This ended up making the most sense to me, but I guess I'll find out how wise this is after about 20K miles 
4) If you ever decide to rebuild your own transmission, I can offer this: You really don't need any special tools, if you're very clever. I took a safer route with shim measurement and input shaft bearing race extraction, but that's just me. I HIGHLY recommend using a shop press to disassemble and reassemble the input and output shafts. I also used a long jaw puller on the input shaft spring in conjunction with the press to disassemble and reassemble the input shaft.
5) I used both the Clymer and BMW factory manuals for reference and to create my strategy. I read both manuals several times (the transmission and clutch sections) before ever even starting the project. I also spent several hours online reading anything I could find to offer some hints that might be helpful. I didn't find many. There were a couple of YouTube videos that didn't really help much, because they didn't show solutions for the toughest problems, mainly because they didn't do re-shimming or any bearing replacement. They, therefore, didn't have to deal with heating the cases to make the new bearings fit. (I didn't need to heat anything to disassemble the transmission. Everything came apart nicely without the need to heat the case or the cover. But the new bearings didn't just slide in and required heat. A heat gun was enough to do the job.)
6) I built a special stand out of wood to hold the rear of the bike up after removing the center stand. I highly recommend you spend some time figuring out a strong stand for this. You will end up moving the bike around a lot (e.g. torquing the clutch nut to 140Nm) and need to have some stability.
7) Replace all circlips! If you're going to spend hundreds of dollars on a rebuild, it just doesn't make sense to reuse old circlips. They're one of the few BMW parts that cost less than a dollar. This is especially true inside the transmission, because hopefully you'll only have to rebuild it once
Anyway, here is a link to some images: http://bartosh.us/k75s. It was a very interesting project. My K75 is running so much better now. All of the idle noise is gone. The shifting is crisp and exact. Every day, as the transmission runs in a little bit more, it gets even smoother. I started out with BMW 80W90 Hypoid gear oil, but will most likely try some 70W140 synthetic the next oil change just to see what kind of a difference that makes.
If you've ever rebuilt a transmission like this yourself, please share your experiences and add to this thread. I know I left out a lot, but I wanted to at least post some basics and encourage anyone else out there who is thinking about rebuilding their own transmission, or even just doing a spline lube to go for it. It's not really a job for a novice tech, but even if you have someone else do it for you, it's a good idea to know what's going on at least.
Cheers,
Ty
Updated 5-Jul-2011: Added link to images
I thought the paralever was supposed to help prevent some final drive issues. At one point, I was considering retrofitting a paralever to my K75S, but when I saw my final drive and drive shaft splines were in good shape, I decided to wait.