The pinion is of two-piece construction, the two pieces being identical and nestably mated to define a recess, therebetween, which is interrupted by abutments, and which confines an abutment-engaging pin. Each piece has a plurality of teeth projecting therefrom, and the pieces are so mated that the teeth pluralities project form opposite sides of the formed pinion. A generally U-shaped rack, having teeth formed on the confronting surfaces of the parallel limbs thereof, is in mesh with the pinion, and the pin penetrates an input shaft upon which the pinion is mounted. As the shaft is turned in one direction, the pin drivingly engages the abutments of only one pinion piece, to cause the teeth of the latter to drive the rack in one, given direction. Too, if the shaft is turned in the opposite direction, the pin engages the abutments of only the other pinion piece, to cause its teeth to drive the rack in the same, one, given direction.