Can't find a reasonably priced used 20 spline drive shaft but found a shaft for a '91 16v. Spline tube on pinion end looks too short. Will it fit the '87 20 spline drive box?

The '87 is a Monolever and a 16V will be a Paralever rear end. They are different driveshafts. 26 11 1 454 063 is the part number of the DS you'll need.

If you know some of the details of the bike you're buying a DS from, be sure it's post the 2nd month of '86 production date, forward. It can be from either any K75 or any K100 8V, no matter.

Great! Thank you!
Looks like I need to keep shopping.
Got a great deal on a 20 spline upgrade rear drive box but kinda wishing I'd held out for the "paralever upgrade".
If I have to pay for a new DS it will cost more than the bike did!

If you go for the Paralever final drive "upgrade" you're gunna also need a PL swingarm, PL driveshaft, longer PL rear shockie, torque arm and gearbox 'cause it has the flange to attach the torque arm to. Now your wheelbase is about 50mm longer due to the more lengthy swingarm of the Paralever-equipped models, so you might also need (in order to get the steering geometry and wheelbase back to snuff) a K11 front end, though some have done the swap without the newer front end. The bike will feel as if on rails.........................................

You don't NEED a K1100 front end to go with a paralever rear upgrade. MY K75 turbo has a K75 front end and paralever rear and rides just fine. (Once I redid the front brakes.)

That's right, hence the use of the word might preceding need. It helps to know what you're influencing when you change something. The wheelbase length is considered reasonably important, most especially when one end influences the other so much. The train reference speaks to these things being innately straight line stable anyway and with a longer swingarm it's even 'better'.

If I don't sell my 'Marzoc' front end then I'm going to drag it home in pieces and whack it onto my older K-RS anyway. I look forward to the quickened steering feel. Some would say it's too quick and I'll tank slap my way down the tarmac, uncontrollably, but I've seen it and heard about it being done with good result.

My K75 paralever Frankenbrick has a K1100 front end (done mostly for the brakes). Since the steering head stem has a smaller diameter it no longer has the fluid block damper. The only difference I noticed was that at extremely low speeds (stop and go traffic) that the front end was a little "twitchy." However, this went away when I switched from bias ply to radials.

Although I have actually had a tank slapper on a stock K75RT (despite the fluid block damper and under very unique circumstances that were mostly my fault) I don't think a K11 front end in conducive to tank slappers. My Frankenbrick's steering seems very well-behaved.

Also interesting is that the same front end on the RS 4V has a steering damper while the K11RS does not and I've never heard of a tank slapper on a K11RS. This leads me to the conclusion that the steering damper on the RS 4V is just dead weight.

If you believed 'The Mason' (remember him?), you'd be knocking on death's door the moment you initiated entry into a roundabout, with a K11 front end on any earlier K...he had it happen coming off the autobahn so he knew the danger, and he'd tell you firmly that it's not to be done.

However, if one rides bikes of other brands, even other model BMWs, than K's on a regular basis they see that there are all sorts of formula for creating healthy steering geometry; one that's able to turn quickly (but not too) and be stable (but not fight your turn) at the same time.

My reasons for doing it are a) the 305mm brakes, b) the 17" wheel, c) the choice of tyres expands and, d) to try it for the sake of it. It can always be put back to stock.