Abstract: The invention relates to a central shaft (7) with desmodromic channels, coaxial and internal to a driven shaft (1), slidable in the two directions and exhibiting a plurality of longitudinal grooves (8) inside which, for every second cogged wheel (2) a corresponding plurality of heads (9a) of pivots (9) are housed and blocked, which pivot heads (9a) are destined to exit, with their free end and on a longitudinal sliding action of the said central shaft (7), from corresponding radial seats (10) realized on the said driven shaft (1); to key each single cogged wheel (2) to the said driven shaft (1), locking means (11) are envisaged in rotation, arranged on each second cogged wheel (2) and acting on each pivots (9) which exits.