Belt drive assembly for driving accessory parts of an internal combustion engine comprising a reversible electric machine, comprising a belt with a plurality of longitudinal ribs alternating with grooves and with a plurality of cavities spaced at equal distances from each other longitudinally and obtained transversely with respect to the ribs in such a way as to interrupt them; the pulley comprises a plurality of ribs suited to engage the grooves of the belt and a plurality of teeth transverse with respect to the ribs, spaced at equal distances from each other on the circumference and suited to engage the respective cavities of the belt. The cavities of the belt have a depth at the most equal to the height of the ribs of the belt itself, and the ribs of the pulley have a radial height larger than the teeth, so as to cooperate with the belt continuously along the whole winding arc.