The metallic fitting, e.g. the housing of a shut-off valve, has a boss containing an internal thread ending short of the free end of the boss at a shoulder. An annular recess between the shoulder and the free end contains the insert which is press-fitted into position. The insert is made e.g. of PTFE and has threads of the same pitch as, but pref. of a lesser diameter than, the metallic thread so as to seal on to a pipe screwed into the boss. The free end of the boss is beaded over to retain the insert and the internal edge of the beading may form the first thread leading into the boss.
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