GearWrench 82857N 9/16 x 18 NF Plug Tap The GearWrench 82857N 9/16 x 18 NF Plug Tap features tapered cutting edges, which give a less aggressive cut than plug taps in small diameters. Manufactured from carbon steel, the GearWrench 82857N 9/16 x 18 NF Plug Tap has clear laser-etched size markings for easy identification.