Abstract: A downhole pump has spaced upper and lower pistons connected together by a common rod so that the pistons reciprocate within a cylinder. The cylinder is separated into an upper and lower engine chamber by a frame supported from the cylinder wall and arranged laterally with respect to the longitudinal axial centerline of the connecting rod. A bushing is positioned centrally of the frame and sealingly engages a reciprocating marginal length of the connecting rod. An exhaust port is formed through the frame in spaced relationship to the bushing. A resilient exhaust valve element surrounds the bushing and includes a lower, circumferentially extending edge affixed to the frame outwardly of the exhaust port and further includes an upper seal end which sealingly engages a marginal external surface of the bushing.