Abstract: In a hammermill or the like and employing hammerheads with sockets which receive the rotor hammer arms, slots are provided in the hammer arms to receive cross pins which extend through apertures in the hammerheads and the slots to hold the hammerheads to the arms. Spacers having a convex outer surface which is complementary to the curved slot ends and a concave inner surface complementary to the curvature of the pins are employed to positively position the pins in the desired position in the slots and thus, by use of different size spacers, enable adjustment of the radial extension of the hammerheads with respect to the arms to compensate for wear and maintain a desired clearance with the working chamber wall and thus maintain a desired particle size.