A cam actuator for selectively moving cam elements axially between distinct operative positions in a circular knitting machine, the actuator including a series of adjacent pistions arranged along the axial line of displacement, and stop means for limiting the axial stroke of each piston in the direction of extension. Each piston co-operates with adjacent pistons in the series such that displacement of a given piston in the extend direction causes the remaining pistons located between the cam element and the given piston to be displaced in the extend direction by the stroke length undergone by the given piston.