Abstract: In a mini C-arm x-ray imaging system in which an x-ray source and detector are mounted at opposite ends of a C-shaped track which is supported for sliding orbital movement along its axis of curvature for positioning the source and detector in relation to an object to be imaged, a locking mechanism including a brake shoe bearing against the track, a screw and nut carried by the track-supporting structure for moving the shoe toward and away from the track, a resilient bias acting between the shoe and the screw to urge the shoe toward the track, a handle for turning the screw to move the shoe, and a detent acting between the handle and the support structure to provide a plurality of discrete settings for the handle, respectively representing incrementally different forces exerted by the shoe against the track.