Self-centering unit for tire removal machines, comprising a plate provided with a series of angularly equidistant radial slots, in each of which a clamping jaw is received and slides to grip the edge of a wheel rim, said clamping jaws being linked together in such a manner as to be always equidistant from the axis of said plate, at least one clamping jaw being associated with actuator means causing it to translate in a radial direction by means of a positioner device arranged to vary the working position of said clamping jaws relative to the actuator means, without modifying their travel stroke.