Abstract: A piston assembly includes first and second pistons, a transition arm coupled to each of the pistons, and a rotating member rotating in a first direction about a first axis. The transition arm includes a drive member coupled to the rotating member off-axis of the rotating member. A first counterbalance is mounted to rotate in the first direction, and a second counterbalance is mounted to rotate in a second direction opposite the first direction. The second counterbalance rotates about a second axis offset and parallel to the first axis. Alternatively, the second counterbalance rotates about the first axis. A method of counterbalancing a piston assembly includes mounting a first counterbalance to the piston assembly to rotate in the first direction, and mounting a second counterbalance to the piston assembly to rotate in the second direction.