I have a spare K100 gearbox. No idea of its history. Any suggestions as to how I check if its any good. Its loose, not on a bike.

You would really have to lift the front section off and inspect the dogs and selector forks. The main things that goes on them is the input spline and grub screw on the selector shaft. You can see the spline and the grub screw will show up as a very sloppy shifter.

Input spline looks good. Anything tricky about pulling the front off the box?

There are two shafts about 12mm dia and are the ones the selector forks slide on try to keep them coming away with the front. you will need to lift the lid about 1/2in then you can see the shafts and get a blade at them to prevent them moving with the lid. The Haynes explains the gearbox fairly well and if you haven't got one get it here https://www.dropbox.com/s/1duzdms9c23wjpk/K100%20Workshop%20Manual.pdf

RicK G wrote:There are two shafts about 12mm dia and are the ones the selector forks slide on try to keep them coming away with the front. you will need to lift the lid about 1/2in then you can see the shafts and get a blade at them to prevent them moving with the lid. The Haynes explains the gearbox fairly well and if you haven't got one get it here https://www.dropbox.com/s/1duzdms9c23wjpk/K100%20Workshop%20Manual.pdf

Thanks Rick. I'll drop it apart over the weekend and see what I find.

Ok new question. Are the splines on the output shaft on the gearbox the same for all models. Reason for asking is that I have a good early final drive (86) that I want to put on a later bike (91). Since the splines are different on the final drives can I swap the drive shafts and still use the older final drive.  

You have a 16tooth final drive 1986 and its 16tooth drive shaft 1986 and they will go on your 91 model gearbox as the output shaft on the box all are the same number of teeth at that end, if you think your 20tooth drive shaft is getting worn then keep it and when cash allows send it off to be repaired, moto hansa does it in sydney, so surely Brissy has someone doing spline replacement as well.

Not sure if that's 100% true about 86 bikes.  I bought one that had the 16 spline setup, not the 20. From the miles (59K) and general maintenance condition of it I think it was 16 from the factory.  But who knows, somebody may have swapped sometime during its lifetime.

Bottom line though is that you can swap the drive shaft and final drive if both have the same number of splines.  For example, I put K75 32/10 on that K100 to give it some more torque.  

Edit: And, while we're on this topic, I don't understand why BMW went from 16 to 20 as, at least from what I've seen, the 16 spline setup seems much less prone to spline wear.

No worries then, I'll put both the 16 tooth driveshaft and final drive on. The old one (20 t) is getting a bit worn so its time for a change.

More torque can be transmitted through a 20 spline than a 16 of the same diameter but to make it work you need a uni joint both end not one end.
The Kawasaki z1300 or KZ as you know them had much the same problem but at least you were told to lube them.

RicK G wrote:More torque can be transmitted through a 20 spline than a 16 of the same diameter but to make it work you need a uni joint both end not one end.
The Kawasaki z1300 or KZ as you know them had much the same problem but at least you were told to lube them.

This makes no sense to me.  Each set of splines will be able to deliver the same amount of torque until the splines reach their shear limit and fail - which they never will.

On a 16 spline the spline is deeper and so the shaft is smaller in diameter and a 20 spline has more contact area overall than a 16 spline so when both are taken into consideration a 20 is stronger than a 16 and in theory longer lasting but first of all you need to lube them properly to have ANY benefit at all and as we all know BMW really stuffed up on that one.