Abstract: A union nut, for cooperation with a first tube having an external thread and a second tube having a radially outwardly projecting flange, has an annular body with an internal thread to engage with the external thread of the first tube, the body being made of a synthetic resinous plastics materials of adequate strength to resist forces exerted by tightening of the nut, and a tool-engaging abutment element secured releasably on the body and at least partially projecting radially inwardly beyond the internal thread of the body so as to be able to abut axially against a flange on the other tube. Tightening of the nut onto the threaded tube draws the two tubes axially together. The abutment element is made of a material, e.g. stainless steel, selected to have adequate strength to resist bending or fracture under the axial forces applied, and to resist damage by the application of a tool to the nut.