A piston has a pair of diametrically opposite skirt portions with convex outer surfaces. Each skirt portion has opposite side edges extending substantially parallel to a central axis of the piston to a free end. The skirt portions each have a recessed slot extending radially into the convex outer surface to a recessed surface. The recessed surface extends fully across the convex outer surface to the side edges and separates an upper band section of the convex outer surface from a lower band section of the convex outer surface.