Abstract: The invention provides a method of joining two rigid tubes such as are used in drill stems wherein a nipple internal to the tubes is threaded and screws into at least one of the tubes, the nipple provided with a zone of reduced internal diameter which registers with a cavity in the internal bore of the tube and the zone of reduced internal diameter expanded outwards into contact with the tube to occupy the registering cavity in the tube and form a positive lock against tensile forces.

Abstract: A coupling thread structure for percussion drill elements including a rod (10) having an external thread (14), and a sleeve (24) having an internal thread (26) with the threads, when the sleeve and rod are coupled, having abutting (16) and non-abutting (18) flanks and with the flanks being joined by bottom (22) and crest (20) portions, wherein the threads have at least two starts; the abutting flanks (16) are substantially straight along their whole abutting contact portions (CL) and form an angle of between 10.degree. and 25.degree., preferably 15.degree. to 20.degree., with the drill axis; the pitch angle of the threads is in the range 9.degree. to 20.degree., preferably 11.degree. to 16.degree.; the crest portions (20) are substantially straight and intersect the abutting flank (16) portions at a well defined edge (30); the non-abutting flanks (18) have a flank angle which is considerably greater than that of the abutting flanks (16); the flank angle of the non-abutting flanks (18) is in the range 50.