Extension or Coupling Nut Used for set up work on machine tools. Made from cold finished steel, case hardened with a black oxide finish. S&W P/N Thread (A) Hex Size (B) Total Length (C) HN-0 1/4-20 9/16 7/8 HN-1 5/16-18 9/16 7/8 HN-2 3/8-16 11/16 1 1/8 HN-3 1/2-13 7/8 1 1/2 HN-4 5/8-11 1 1/16 1 3/4 HN-5 3/4-10 1 1/4 2 HN-6 7/8-9 1 3/8 2 1/4 HN-7 1-8 1 1/2 2 1/2