Abstract: An impact type rock crusher having a rotor comprised of a plurality of stirrup type hammers adapted to swing through arcs of approximately 180.degree. between bumper blocks about pivot axes as well as rotating about the main shaft of the rotor. The grate positioned beneath the rotor through which the crushed material exits may be moved uniformly radially with regard to the rotor to an alternate grate position to vary the clearance between the hammers and the top surface of the grate. The overall design of the system reduces the height of the apparatus, and the apparatus may be made portable for movement from one location to another.