Abstract: The selector rings (9-11) of a gearbox are moved axially of the mainshaft (8) to engage and disengage gears (1-6) by respective pairs of push-rods (15, 16) which extend inwardly between the gears (1-6). The push-rods (15, 16) bear on oppositely-inclined faces (22) of pairs of blocks (17) which slide in slots (18) parallel to the shaft (8) and which engage respective rings (9-11) at diametrically-opposite locations. Actuation of one pair of rods (15; 16) causes their blocks (17) and the engaged ring (9-11) to move in one direction whereas actuation of the other pair causes movement in the opposite direction. The rods (15, 16) slide within mounting blocks (19) inset between pairs of the gears (1-6), and are actuated by the cam surface (26) of a longitudinally-driven rack (25). Alternatively, the push-rods (47, 48 FIG. 8) are actuated by individual hydraulic rams (45, 46).