The 4 Bolt Flange is a known leak-free connection.  It is specifically designed for use in high pressures, larger pipes and tight quarters applications.  They are commonly used to connect larger sized hose and tubing in fluid power systems.   Standard Features -- Threaded x O-ring Face: Code 61 (3000 PSI) NPTF Thread - Series W43 NPTF Thread Flat Face - Series W104 NPTF Thread Reducing - Series W43 -12 SAE Straight Thread - Series W46 Code 62 (6000 PSI) NPTF Thread - Series W44 -12 SAE Straight Thread - Series W48 Standard Features -- Socket x O-ring Face: Code 61 (3000 PSI) Flat Socket Weld - Pipe - Series W4 Flat Socket Weld - Pipe Flat Face - Series W61 Flat Socket Weld - Pipe ** 500 PSI - Series W72 Code 62 (6000 PSI) Flat Socket Weld - Pipe Flat Face - Series W60 Standard Features -- Gage Port Spacer: Code 61 (3000 PSI) - Series ASX Code 62 (6000 PSI) - Series AS Includes O-ring and Plugs Standard Features -- Blanking x O-ring Face: Code 61 (3000 PSI) - Series W36 Code 62 (6000 PSI) - Series W38 Standard Features -- Elbow: Code 61 (3000 PSI) NPTF Thread - Series W168 Socket Weld - Series W176