Our range of brass threaded tank outlets are truly a work of art. Beautifully made and Australian Watermark Approved, in my opinion, these are the very best tank outlet that you can buy. All sizes have a male outside thread on one end and a female BSP thread the next size down on the inside of both ends. The locknut is a reverse thread so that it does up nice and tight on the thick rubber gasket and, when you are screwing on the ball valve, the fitting remains tight and waterproof. This is the 20mm male BSP end with 15mm female BSP threads on the inside of each end. For this fitting you will need to cut a hole that is 31mm in diameter.