of the shift dog 9 this shift dog includes two external cylindrical portions 9a and 9b which do not have the same diameter the front portion 9a of smaller diameter includes a smooth edge designed to come to a stop against the counterbore 23 provided on the first member 8 the edge 24 which separates the front and back portions of the shift dog limits the forward stroke of this shift dog the walls of the dog clutch bore 25 include helical gears or spiral teeth 19 which are designed to mesh with the grooves 17 of the piston the reverse stroke of the dog clutch is limited by the back edge of the back portion 9b which includes numerous teeth 26 the axial displacement of the dog clutch causes either one of the two following actions to occur the gears or teeth 26 mesh with the complementary gears 27 which are cut in the front end of the fixed part 10 this fixed part also includes a central bore 28 to permit the piston 7 to reciprocate therein as shown in fig7 and 8 as the dog clutch is driven forward the gears 26 are disengaged from the fixed gears 27 the crown or.