Abstract: An adjusting mechanism is for position-adjustable connected structural parts, such as in a driving-belt tension mechanism with a clamping coverplate. In the mechanism a first structural part that must be adjusted with respect to a second structural part that has a rack or gearing, into which a pinion that, by a threaded member, is clamped to position both structural parts, engages in such a way that, when the screw connection is loose a mutual shifting of the parts can be achieved by turning the pinion. The pinion has a nut-like collar that has a rack-meshing profile and an inside profile which is in one embodiment loosely slipped about a screw of the threaded member, and in another embodiment, rotatably fixed relative to the screw of the threaded member.