Abstract: An cam adjusting device (30) comprises a first and second element sliding reciprocally and a cam (32) pivoted to the first element by means of a pivot (33) rotating around an axis (48) to react with one of its lateral surfaces (35, 38) against a bearing surface (34, 37) on the second element, upon rotation of the cam around the axis thereby achieving sliding and adjustment of the reciprocal position of the first and second element. The cam is shaped so that, for any angular position of the cam in its active stroke the centre of rotation of the cam is substantially in the region of a straight line (41) passing through a point (36) of contact between a lateral surface of the cam and a bearing surface of the second element, said straight line being slanted at an angle .phi. identical to the angle of friction in the point of contact. A cam meeting this condition has been found maximize the amplitude of the adjustment without encountering instability.