Abstract: A size reduction machine for reducing the particle size of material, comprises a frame, an arm pivotally connected intermediate its ends to the frame, a counter weight movably connected to one end of the arm, a motor having a drive shaft mounted on the other end of the arm, an impeller connected to the free end of the motor shaft, an enclosure carried by the frame below the motor, the enclosure having fenestrated side walls and an open top for receiving the motor shaft and impeller, the motor and impeller being bodily movable into and out of the enclosure upon pivotal movement of the arm, and an adjustable stop positioned between the frame and the arm for limiting the maximum travel of the impeller into the enclosure.