a tool for mounting a belt in a groove of a pulley of an automotive vehicle wherein the pulley has a rim and a flat lateral surface on each side of its groove . the tool includes a main body having an internal flat body surface and an external flat body surface ; a flap provided on an edge of the main body and bent substantially at a right angle relative to the internal flat body surface and in the direction thereof . the tool also has a belt guiding area having substantially an arcuate shape and projecting substantially normally from the external flat body surface . the tool is useful for mounting a power transmission striated belt that includes an elastomeric matrix and a lengthwise supporting structure consisting of polyamide 4 . 6 twisted strands .