A coupling for sealing an outer cylindrical wall of a member of a fluid flow system extending along an axis. The coupling has a body and nut encircling the cylindrical wall which are configured to receive either a conventional O-ring or packing. The body includes a recess having a wall surface tapering at an angle in the range of 7.degree. to 20.degree. relative the axis and cooperates with the nut and cylindrical wall of the member to define an open area for receiving either an O-ring or packing.