The F800ST presents a few dimensional challenges for caliper mounting Motolights. After lots of trial-and-error engineering, here is what‘«÷s required for the F800ST caliper mount Motolight installation as shown:
10-695 Caliper Mount Spacer Block Set
10x1.25x60mm socket head zinc plated 12.9
10mm lock washer
The 10-695 spacer block has a 25mm offset and the stock BMW caliper bolt is 35mm, therefore a 60mm bolt is required. Please note that I did not use the lock washer because using it requires an additional 2-3mms bolt length (63mm?) to fully penetrate the caliper. I believe with proper torque, the clamping force should be sufficient. If you purchase a kit online, confirm it includes these parts.
More pics: http://www.pbase.com/bricciphoto/motolights