A fitting widely used in petroleum marketing operations.
To understand what a double-tapped bushing is, and how it works, begin with an understanding that the standard openings in an American underground storage tank is 4 inches in diameter. Typically the piping that carries product from a tank to a dispenser is 2 inches in diameter, as is the suction stub inside the tank. Two 2-inch pipes, then, must meet in a 4-inch opening.
A double tapped bushing provides the method for accomplishing this. The bushing is a flat, round fitting, about 2 inches thick and 4 inches in diameter. It is threaded on its outside circumference. In the center of the fitting is a threaded hole, 2 inches in diameter. This center hole is threaded in both directions, from the top and from the bottom of the bushing.
The bushing is installed in a tank opening, with a suction stub screwed into the center hole from the bottom. The connecting dispenser piping is screwed into the center hole from the top.
Because the center hole is threaded (tapped) both from the top and the bottom, the fitting is referred to as a double-tapped bushing.