Installed fuel indicator, and drilled the 5 mm holes in fuel-tank. Than cut the threaded washer and twist it to make it possible to screw washer to inside tank. Insert rubber washer with Loctite 577 on both sides. To hold washer in position I used two extra long M5 screws, than installed correct size screws in all positions. Finally I drilled a 18 mm hole in lower fuel-tank panel, the position of drain-valve.
