I sit and make these little babies in-house, sitting in front of the lathe for a rediculous amount of time. They are matched to the weld-in bung and are perfect for a bit of engraving...and are a nice touch even if you're not using a bunch of brass.
* the opening is just shy of 1", enough for the pump nozzle
* OD - 1-1/2""
* hole to drill in the tank - 1-3/8"