An impact crusher, comprising a frame defining a crushing chamber, a rotor (8,208) provided with at least one hammer, at least one impact apron plate, and a rotor positioning device configured to adjust a rotational position of the rotor (8,208), characterized in that the rotor positioning device comprises a piston motor (16,216) coupled to the rotor (8,208) and having: a cylinder block (18,218) comprising a plurality of cylinders (20,220); a plurality of pistons (22,222), a respective piston (22,222) being slidably mounted in a respective cylinder (20,220); and a surface (24,224) against which the pistons (22,222) are configured to exert a force; wherein the cylinder block (18,218) and the surface (24,224) are arranged for relative rotation, and wherein the cylinder block (18,218) or the surface (24,224) is operatively coupled to the rotor (8,208), such that relative rotation of the cylinder block (18,218) and the surface (24,224) causes rotation of the rotor (8,208); the rotor positioning device being operable in two modes, wherein: in the first mode, the piston motor (16,216) is configured to actuate the pistons (22,222) in a predetermined sequence such that the pistons (22,222) cooperate with the surface (24,224) to generate torque, thereby causing relative rotation of the cylinder block (18,218) and the surface (24,224), causing rotation of the rotor (8,208), and in the second mode, the piston motor (16,216) is configured to arrange the pistons (22,222) and/or the surface (24,224) such that free rotation of the rotor (8,208) is permitted. the rotor positioning device comprises a piston motor (16,216) coupled to the rotor (8,208) and having: a cylinder block (18,218) comprising a plurality of cylinders (20,220); a plurality of pistons (22,222), a respective piston (22,222) being slidably mounted in a respective cylinder (20,220); and a surface (24,224) against which the pistons (22,222) are configured to exert a force; a cylinder block (18,218) comprising a plurality of cylinders (20,220); a plurality of pistons (22,222), a respective piston (22,222) being slidably mounted in a respective cylinder (20,220); and a surface (24,224) against which the pistons (22,222) are configured to exert a force; wherein the cylinder block (18,218) and the surface (24,224) are arranged for relative rotation, and wherein the cylinder block (18,218) or the surface (24,224) is operatively coupled to the rotor (8,208), such that relative rotation of the cylinder block (18,218) and the surface (24,224) causes rotation of the rotor (8,208); the rotor positioning device being operable in two modes, wherein: in the first mode, the piston motor (16,216) is configured to actuate the pistons (22,222) in a predetermined sequence such that the pistons (22,222) cooperate with the surface (24,224) to generate torque, thereby causing relative rotation of the cylinder block (18,218) and the surface (24,224), causing rotation of the rotor (8,208), and in the second mode, the piston motor (16,216) is configured to arrange the pistons (22,222) and/or the surface (24,224) such that free rotation of the rotor (8,208) is permitted. in the first mode, the piston motor (16,216) is configured to actuate the pistons (22,222) in a predetermined sequence such that the pistons (22,222) cooperate with the surface (24,224) to generate torque, thereby causing relative rotation of the cylinder block (18,218) and the surface (24,224), causing rotation of the rotor (8,208), and in the second mode, the piston motor (16,216) is configured to arrange the pistons (22,222) and/or the surface (24,224) such that free rotation of the rotor (8,208) is permitted.