I've seen much worse, to me there is still a bit of life there if you don't twist too quickly. I picked up a second hand shaft in good condition for 90 quid from Motorworks.
Do you have contact with any of the FB members, you could try and persuade them to post you some of their Staburags spline lube.

 

I have already got me some Staburags. As I said, the rear splines on the shaft look pretty worn, but the drive splines seem to me to be OK. I really would like to have opinions from forum members about the condition of the drive splines. There is only one way to drive the K75: "like you stole it".

If you are going to ride like you stole it then go steal another one for spares.
The splines on the drive are acceptable but not by any means good and the shaft is considerable worse.
Motobins can supply you with a weld on spline replacement part
http://www.motobins.co.uk/bmw-parts.php?model=K%20Series 
I replaced one a while ago and did a rocwall hardness test which was a bit harder than the OEM but not significantly harder and when I set the shaft up in the lathe to remove the spline there was .013" runout and when finished .002" runout so it came up better than OEM.
Any good engineering shop should be able to do it.
As for the drive I would try to get a 2.91 ratio from a K100RT which is a big improvement over the 3.2 ratio of the original K75 and a bonus is better fuel consumption (they are easier to get as well).

I'd not use it. 

Splines are like screw/bolt threads, to work they need the correct interface between the mating surfaces. One that's damaged will soon damage the other.

My guess is that the slight wear you found at the last inspection has resulted in the rapid wear that you now see.

No amount of Optimol paste will stop that happening.

I have a 2.91 drive in my wifes K75RT and it is really good. She gets 65mpg so nearly 400km to a tank and it doesn't sound or feel like iy needs another gear.
I would use it if I had no choice but as Mike said it is best not to start that wear pattern over again.
2.91 gears are easy to find all the K100 RT/LT and K1100LT had them.

I asked my friend in Hamburg, who has been breaking police BMW for many years, if he could help me. 

Here is his answer:
You can easily fit other bevel drives. The police version has the regular 37:11 that means good acceleration and a lower top speed. The Rpm is very high at higher speed. You can use other drive ratios such as 32:10, 32:11 and more. In some cases you might need a 16 teeth driveshaft. We will brake a K100 in January. Shall we get back to you then?


Frank deals on fleebay if you are interested http://www.neumann-werke.de/

There is a 33/11 which is in between the K100RT and your K75 drive but I have a feeling that they were 16 spline shaft.
It is good that you have that contact for parts unfortunately for us in Australia the postage is a real killer.
If you do put a 2.91 drive in it pays to give the engine about 4° more advance at base idle, it gives a heap more torque at low revs and helps overcome higher gearing in 1st gear. I found you need to use the clutch a little more at take off but once you get used to it all is good.

They are different ways of expressing the same concept -- the ratio of input rotations to output rotations.  One is expressed as tooth count (32:11) and the other as the division of those teeth (32 / 11 = 2.91).  Anton's page will give you good insight into the various final drive ratios available for K-bikes & R-bikes. The cool feature of that page is the graphing function that can allow you to compare the engine RPMs for a given speed between different ratios.

TIV, I wouldn't have an issue using your old drive. Yes, there is some wear, but the drive is quite a bit harder than the shaft inner splines. The shark tooth you see inside the shaft is nowhere near what you see on the drive. There will initially be some accelerated wear until both parts meet up over the full contact surface. I'm really surprised that the wear was so fast after greasing; did you find any water in there?

Hi ReneZ. No water in there. Unfortunately I didn´t take pictures when I got the bike and overhauled it about 7.000 miles ago. I might have underestimated the wear at the time, but my feeling is that it has increased, especially the shaft splines.

I've got a used final drive from Motorworks and a used OEM shaft is on it´s way from Canada. It´s in mint condition according to Bruno at Bruno's Machine & Repair. 

I think the splines are in good condition. 




The crown seal is not leaking, but it´s ugly, so I decided to open the F-D in order to clean it and put in a new seal, O-ring and take a look at the interiors.





Everything looks good on the inside, no chipped teeth or anything else that I can see.




No play can be felt in the crown wheel bearing and it spins freely and smoothly.


It seems that he pinion spins smoothly when I turn it by hand and I can't feel any burr or hear any noises from the bearings. 

Oooops!
My photo is gone......

How do I post a photo?..I am using an iPad..
Thanks..

Cheers

Looking good TIV!