In an apparatus for angular positioning of an operating arm of a tire changing machine, the arm has a first or proximal end hinged to a column extending vertically from the base of the tire changing machine. The arm is hinged to the column for swinging in a horizontal plane in a first direction of rotation toward a work position or in an opposite direction of rotation toward a rest position. The arm has an opposite, distal end having a support element for supporting work tools associated therewith. An adjustable lock or stop is provided for locking rotation of the operating arm in the first direction of rotation, the apparatus comprising a second lock or stop for locking rotation of the operating arm in the second direction of rotation, opposite to the first direction of rotation.