A mounting assembly has a machine element with a threaded coaxial bore on a shaft, for rotation with the shaft. A split sleeve having external threads is provided and adapted to be fit into the element's threaded bore and mounted around the shaft at any desired position longitudinally and circumferentially. The threads of the split sleeve have apexes at a uniform diameter when unstressed. The threads of the split sleeve are shaped to cause contraction of the sleeve, when threaded into the element's bore, to secure gripping engagement with the shaft so that it frictionally clamps onto the machine element.