Both the inner and outer circumferential surfaces of a coil spring are peened by passing sequential segments of the spring around offset pulleys mounted within a peening cabinet. As the spring is pulled around one of the pulleys, the coils fan out from the pulley to permit the peening media to impact portions of the inner and outer circumferential surfaces of the spring, and as the spring is pulled around the other pulley, the portions of the surfaces not peened as the segment traveled around the first pulley will be peened.