Abstract: A slider block has an inner bore which receives an eccentric pin, and a generally flat drive surface over a limited circumferential extent to be engaged by a flat drive surface on an eccentric pin. A counterweight is positioned on an outer periphery of the slider block. The counterweight has a center line defining a first plane. A second plane is defined parallel to the first plane, with the second plane including a point on the flat drive surface of the slider block, with the flat drive surface of the slider block including a third plane, and a non-zero angle defined between the second and third planes.