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I don't know the bike or the tank.That said, it looks to me like the o-ring seals in a "circle" inside the "circle" of screws. As the screws are carefully not overtightened, the two surfaces should be drawn together and the depression between screw holes should flatten out.Since I don't know the tank, please tell me if the threaded inserts are open to the inside or blind. Also, how tall (proud) does the o-ring set in its groove? 0.018 isn't a lot of gap. I would expect the o-ring to crush into that amount of space.Possible generic thoughts on sealing:There's a product called "SEAL ALL" that I've used successfully to seal gas tank leaks. I even used it to pot an LED bulb in place at the end of a sight glass (to light up the sight glass). No leaks if used correctly, and I've been able to pry parts loose (very carefully) without distorting either part.You might be able to "boost" the o-ring in its groove by pre-packing the groove with waxed string (like candle wick). The string doesn't make the seal, but it makes the o-ring "bigger".You might make a flat gasket from fuel-proof gasket material (available by the sheet at the auto parts store and a lot of hardware stores) to take up the surface irregularities. Dress it with a little sealer on the tank side and a LIGHT torque on the gasket to produce a flat surface. Then do the plate install against the newly-flat tank.Of course none of this will fix anything if the threaded inserts are leaking or if there's a leak somewhere else pretending to come from the plate. So after sealing as well as you can, seal the known openings of the tank, inject a little air (a couple # should do it) and immerse the tank in water. Any leaks will reveal themselves as bubbles.