hbyrid, excellent work! but to get the filller cap 100% watertight you are still missing one step. I've got two of these filler caps for my sill tanks in the rear wheel arches. when i tested them putting the car at an angle, i realized that they are leaking fuel. they are not tight after all. the same happened when i pressure tested the tanks, the filler caps let the air out. the rubber o ring is too small, either geth a thicker one or take some ptfe string that is used for sealing threads and wrap it around the metal were the o ring sits. smear some fuel resistant sealant on that and put the o ring back on. now it is watertight!
regards
spec