The o-ring there doesn't do much except help route the draining fuel to a common output hose. It does not seal the fuel into the bowl. The steel drain screw has a cone on the end that fits inside a conical hole (seat) in the aluminum bowl, and it is interference fit (it jams in there). If you're lucky, you can find a tiny piece of rock embedded in the aluminum float bowl's cone seat. You can insert a q-tip where the drain screw goes and spin it to snag embedded grit. If the seat is trashed beyond hope, or the threads are stripped, the only "nice" fix is to buy a brand new bowl with its drain screw (they come matched). Under the classification "not nice", there is use of epoxy to glue the drain screw in place. I'm not sure I would want that to let go on me in Death Valley in the middle of August.
