Abstract: A joint for casing tubes comprises a pair of tubes each having an external threading on at least one end, and hollow sleeve having corresponding internal threading on both ends. The internal threading of the sleeve on each end terminates in an annular seat which extends radially inwardly and axially toward its corresponding end of the sleeve, and thus has a frustoconical shape. The end surface of each of the threaded ends of the pair of tubes is correspondingly shaped. The threading on the sleeve and pair of tubes is so configured that when the sleeve and tubes are interassembled the crest of the internal threading of the sleeve will contact the root of the external threading on the tubes, whereas the crest of the external threading on the tubes will be spaced from the root of the internal threading on the sleeve.