Also known as National Gas Outlet (NGO) taps, these tap threads in sizes used for compressed gas cylinders and valves. They're made of high-speed steel and are for use on most metal and plastic. With a square shank, they're compatible with tap wrenches and tap holders for drill presses and milling machines. All have a bottoming chamfer with 1-2 chamfered cutting threads, so they're suitable for threading close to the bottom of closed-end holes.