had one leak for the first time, are the top and bottom rubber the same, how tight do you go ,hand tight or wrench ,before glass breaks ,do you use sealent, fuel lube or dry threads.do you replace ball?
Rubber is not the right material. Had a customers airplane once with rubber gaskets, auto fuel swelled the hole shut and gauge showed fuel left, tank was empty. Cork is what belongs in there.
RAC, like Mike said, original gauges use the cork. You don't want to over tighten them because it crushes the cork gasket and they will leak and maybe lose bits of the gasket into the fuel supply. Just tight enough to squish the gasket down, not tight enough to crush it.
Some replacement gauges, like the Dakota Cub high-visibility ones, use a series of o-rings.
I just put a microscopic amount of engine oil on the cork gaskets to make them slippery so that they didn't tear when tightening fittings down. Ball doesn't stick nearly as easy as it does to fuel lube. worked perfectly