I find it easier to plug the tube before I turn and tap the other end. I usually run a 9/32 drill all the way through the tube to clear out any leftovers from drilling the air mixture holes. Turn the brass for the plugs to 0.284 or so, for a nice tight press into the tube. If you get 9/32 brass rod I'd suggest not taking the drill all the way through the bottom to leave it small for a tight fit. The thickness of the plugs is not critical but don't make them too thin.