Fits F800GS/GSA
Also fits F700,800-F/R/GS/GS-A
Not for the new F850GS with keyless gas cap (that’s model M28006-ARM)

Filter does not have a flange at top, instead it fits on the nozzle inside your gas tank (disassembly required)
Nozzle size 35mm
Filter size 6.5×7.5″
Filter is centered on the nozzle
Filter has an anti-abrasion armor mesh
Filter is held in place with a stainless wire that fits thru the vent holes in the nozzle

The special fuel neck prefilter is designed to be mounted inside the original fuel cap. To install you’ll remove the screws holding the flip-cap in place, fit the filter and retainer, and refit the flip-cap.  Installation PDF: F800GS Installation

But an easier way to understand the installation process is to watch this video.

The HDM3D media filters down to 10 Microns, which will allow a very good protection of the fuel pump and main filter. WATER,  and other contaminants (liquid and solid) will be separated during refueling.

The original BMW fuel filter system has a strainer working between 40 and 60 microns, made from nylon mesh, which is totally inadequate to protect the fuel pump.

This filtration level will both preserve the fuel pump and prolong fuel original filter life, thereby avoiding nasty surprises and high repair costs.

Double-check your bike’s make, model, year, gas tank design, filler nozzle, etc. before ordering. 
Take careful measurements of your filler nozzle and confirm for proper fitment. (see measurements of filter above).
If you order the wrong filter you’re responsible for any shipping fees needed to replace it with the proper filter, plus any price differences.
Filters that have been immersed in gas (or are not in like-new condition) are NOT eligible for return or refund.