What's the thickness of the tank itself? Just 3/16" plates to drill and tap might be a bit thinner than I'd prefer but take a look at a 1/4" or 3/8" NPT brass fitting to estimate how many threads would be engaged. I don't have a magic number of threads that would be a minimum, but I'd think 3 thread engagement should work OK. Probably a better chance of that happening with a 1/4" fitting than a 3/8". Use a good thread "dope" that's fuel resisitant. Since those plates are there, that also might be a good enough thickness to use your rubber grommets. I've never used the grommets but I think the hole size needs to be sized carefully so the grommet is compressed a fair amount when the metal fitting is pushed in to prevent leaks. I'd also think deburring both sides of the hole would help the seal and prevent cutting or marring the rubber grommet. I'm just thinking out loud about what I'd do, so I hope others will post advice that might be based on their experience. BTW...I'd say your welder friend is pretty good at welding aluminum. Lynn |