An agricultural harvesting machine (100) for crop material is disclosed. The agricultural harvesting machine (100) for crop material comprising: a crank arm (142) connected to a rotational power source; a plunger (146) having an extended position which is located further rearward in a compression chamber (140) than a retracted position; and a connecting link (144) connected between the plunger (146) and the crank arm (142), one of the connecting link (144) and the crank arm (142) having a variable length, the movement of the plunger (146) being decoupled from the movement of the crank arm (142) during a startup mode, and the movement of the plunger (146) being coupled to the movement of the crank arm (142) during a run mode.
