a drill string during use of the stabiliser 10 while the keys 24 permit the transmission of drilling torque and rotation through the stabiliser 10 the casing 12 and the mandrel 22 together with both the string couplings 14 and 30 are hollow throughout the length of the stabiliser 10 and mutually sealed to permit drilling mud to be pumped under pressure through the stabiliser 10 when it is incorporated as part of a drill string the radially inner end of each radial spacer 16 has a wedge shaped camming surface in the form of a substantially planar surface inclined at a small angle preferably 5 15 degrees to the longitudinal axis of the stabiliser 10 the mandrel 22 has matching camming surfaces in the form of a series of part conical steps 32 machined in its external surface at substantially the same angle to the longitudinal axis as the inner ends of the spacers each part conical step 32 is circumferentially continuous to obviate any necessity or a specific angular alignment between the mandrel 22 and the casing 12 during assembly of the stabiliser 10 the mutual axial spacings of the steps 32 correspond to the axial spacings of adjacent spacers 16 such that the camming surfaces 32 on the mandrel 22 will simultaneously engage each camming surface formed by the inner end of each of the radial spacers 16 telescopic contraction of the stabiliser 10 between the string couplings 14 and 30 by increased insertion of the mandrel 22 into the casing 12 results in the part conical camming surfaces 32 wedging against the inner ends.