An agricultural harvesting machine (100) for crop material is disclosed. The agricultural harvesting machine comprising: a pre-compression chamber (120) having an accumulation phase in which crop material accumulates until a pre-determined fill condition and a loading phase in which crop material transfers from the pre-compression chamber (120) into a compression chamber (140) when the pre-determined fill condition has been attained; 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, the movement of the plunger (146) being decoupled from the movement of the crank arm (142) during the accumulation phase, and the plunger (146) moving to the extended position following the loading phase; 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.
