A new and improved two-part phase changing device which has an axially movable quill shaft extending through a hollow camshaft and has one end of the quill shaft directly connected to the hub member of the timing gear through straight splines and has the other end of the quill shaft connected by helical splines to the camshaft so that axial movement of the quill shaft provided by a ball nut transmission located adjacent the helical splines of the quill shaft serves to angularly reposition the camshaft a predetermined distance upon actuation of an electric stepper motor.