driveshaft spline counts

[motohippy]

Gurus of motobrickdom...I come to you in confusion.
I recently picked up a (allegedly) '89 k100rs final drive (with ABS ring) with the goal of dropping the highway rpm on my '88 k75.
Went to fit it up, and no dice.
Much to my surprise, the k100rs final drive has 16 splines on the input, compared to 20 on my driveshaft.
The Max BMW fiche shows the same part# for an '88 k75 and a '89 k100rs driveshaft... so what gives?
This raises a second question- if I pick up a 16 spline driveshaft, will it fit my transmission?
And a 3r

[frankenduck]

Gurus of motobrickdom...I come to you in confusion.
I recently picked up a (allegedly) '89 k100rs final drive (with ABS ring) with the goal of dropping the highway rpm on my '88 k75.
Went to fit it up, and no dice.
Much to my surprise, the k100rs final drive has 16 splines on the input, compared to 20 on my driveshaft.
The Max BMW fiche shows the same part# for an '88 k75 and a '89 k100rs driveshaft... so what gives?
This raises a second question- if I pick up a 16 spline driveshaft, will it fit my transmission?
And a 3r

1 - The 16 spline final drive originally came from an 85 or earlier K100.

2 - Yes, if you buy a 16 spline shaft it will "bolt right up" to a K75. (I even know someone who has a 16 spline K100 setup on a K75.)

3 - The 16 spline drive shafts wear less than the 20 spline shafts. Lord knows why BMW made things "worse" by going from 16 to 20 splines.

[Laitch]

The Max BMW fiche shows the same part# for an '88 k75 and a '89 k100rs driveshaft... so what gives?

Their assemblies are both 20-tooth. The 16-tooth has a different part number was replaced at 02/86 by the 20-tooth, according to MAX. Your replacement could have been taken from a later model moto that had an earlier model's rear drive and driveshaft swapped into it as a replacement for some reason. It's not an uncommon event and some salvagers might not note the difference unless they worked with a wide range of Brick model years. As frankenduck indicated, either type of driveshaft's universal joint will clip to the transmission output shaft.

[motohippy]

Thanks... at least it's not just all in my head. 
Looks like it's time to hunt down a new driveshaft...