Abstract: An attachment for mounting on a rod or shaft when an end of the rod is not free, in the form of a split collar, pulley or gear, is in two parts having half-cylinder inner portions. The parts may be mounted on diametrally opposed sides of a rod or shaft when axially displaced from one another. Each part has a pair of interlocking elements. Each of the pair of interlocking elements of the first part interdigitates and thereby interlocks with one of the interlocking elements of the other part when the two parts are moved together axially to form a completely encircling attachment on the rod which resists radial dismemberment. A threaded passage extends radially through at least one of the parts to receive a long threaded fastener such as a thumbscrew to fixedly engage the rod. The attachment may be readily and rapidly mounted, locked and removed by an unskilled user without the use of any tools.