A drive unit comprising a transmission with two coaxial bevel gears (11, 12) fitted opposite one another on the same engine shaft (10), which engage a bevel gear (13) fitted to a shaft (14) perpendicular to the preceding one, a device designed to mesh the engine shaft with one or other of the bevel gears, constituted by clutches (20, 21) housed in the body of the bevel gears (12, 11) and controlled by a pressurised fluid, wherein pipes (24, 25), designed to convey the pressurised fluid, are formed in the engine shaft (10), wherein, in order to supply pressurised oil to the pipes (24, 25), an axial inlet pipe (34) is formed in the head of the engine shaft (10), which the axial pipe (34) feeds a distributor (35) constituted by a valve that slides on the engine shaft, which the slide valve (35) can move between two positions in which it places the input pipe (34) in communication with one of the pipes (24, 25).
