Mw074 and I exchanged a few private messages and determined that there are actually two different mounting pieces for the Z racks.
If you look at the parts fiche where the Z racks and hardware are located ("Set Case Holder"), you will notice part number 18 21 1 454 982, "rubber mounting".
It looks like this:
For ease of reference, we'll call this part "982". If you have Z racks already installed on your K, this is almost certainly what you have on your bike.
If your K bike did not originally have Z racks installed on it (which is likely if you are reading this at all), the piece that is already on your bike will probably look like this (and hopefully not as rusty):
That one has a slightly different part number -- 18 21 1 454 98
1 ("981").
What's the difference? The only difference is that 981 has slightly longer prongs, whereas 982's are slightly shorter. How much shorter?
5mm. So, if you're like me and you finally got your Z racks, saddlebags and you're keen to get it all set up and you find that the bolts that go through the "rubber mounting" are not long enough, you will have two options:
(1) buy a 982, either used or from a dealer; or
(2) grind 5mms off of the prongs with a bench grinder.
I chose (2), and it worked perfectly. I apologize for mixing metric and imperial here, but you can do this by either grinding 5mm off of each prong measuring from the top down, or you can grind them until the total height (including the back plate) is 3/4". Either way, this will leave the necessary room for the thickness of the Z rack itself to fit between the mounting and the foot peg plate.
I hope that this helps someone else in the future. I tried to find information about this, and I couldn't. So, maybe this is a new thing that mw074 and I found out? How exciting.
