Ive been looking, waiting for someone to post about this, but no one will so here it is. My F800GS now has an extra 4 gallons of fuel on board, relatively low cost installation. Tour tank onboard. First step was to aim a drill at my $900 tank and maybe ruin it, I really wanted to do this with my eyes closed but I just bit my cheek and drilled. This will be the entry point to refill the main tank on the fly. If you look closely at that second picture you can see the tank is double walled, I was a bit worried about sealing the bulkhead fitting because of that but so far, so good. Why there? Well, it seemed to be an easy place to get to without dismantling the bike, but mainly I needed to be able to reach the inside nut on the bulkhead fitting with a wrench. Before I drilled I stuffed a towel into the tank to catch any drill shaving that fell inside the tank, worked, they were all right there and easy to clean out. Next I had to be able to start the interior washer and nut with out dropping them into the tank, so I threaded a zip tie through the bulkhead fitting and slid the washer and nut down the tie, no way they could miss their target, then it was easy to reach in and get the nut started. I coated the Teflon washer with gas proof thread dope before putting it in. I was worried about a leak there and it is holding well so far. Bulkhead fitting installed, it looks somewhat exposed here but is protected by the luggage rack and cases when finished. Gas line and luggage rack installed. You can see how I mounted the tank here. I found a sheet o f aluminum, bolted the tank to it. The plate is secured in the back by the two rear bolts that hold the grab rail on, the front I bolted a 1/8" x3/4" steel bar to the plate, bent loops to wrap it around the grab rail and clamped it in place. It is real solid. Left side view. Valve is on this side, where it is located is easy to reach while riding. When the distance to empty gets below about 40 miles I open this valve and watch the DTE mile increase.