Abstract: The boom of an earth-working implement, such as a backhoe, is swung laterally to-and-fro through a substantial arc by mechanism that requires only a single, double-acting, fluid pressure piston and cylinder assembly. A shaft on the boom is journaled onto the frame of the implement and carries a pair of superimposed sprocket wheels that are partially wrapped in opposite directions by a pair of drive chains. The chains are anchored to opposite ends of a bar reciprocable transversely of the axis of swinging movement of the boom, and the bar is coupled with the piston and cylinder assembly such that when the latter is actuated, the bar reciprocates to wrap one of the chains about its sprocket wheel while the other is unwrapped from its sprocket wheel, thereby effecting powered swinging of the boom.