A locking system for directly clamping the tractor assemblies (16) in a printer to the stationary platen (14) in order to prevent any relative motion of the tractors with respect to the platen which may be caused by a lack of straightness of a drive bar (22) which drives the tractors. The locking device includes an L-shaped lever (26) which is pivotal so as to clamp a rearwardly extending portion (24) of the platen between the bottom of the tractor (16) and the top (26a) of a leg (26a) of the lever. The device may include a cam action locking knob (30) which positively locks the lever (26) in its clamped position. Both the lever and the knob are made of molded plastic. In order to balance the clamping force which is applied, a lever (26) may be connected to each side of the tractor (6) and actuated by a pair of connected cam action knobs (30). Several alternate locking systems are also disclosed.