Abstract: A compressor utilizing opposing cylinders and flap valves in which the configuration may be readily modified from compressor to vacuum operation by simple reversal of components. Convertibility is facilitated by a valve plate having apertures which connect opposed cylinders and a manifold channel to opposed end chambers. Reversible flap valves are mounted on the valve plates and on the pistons. Additionally, a Scotch-yoke type mechanism of novel construction is provided in which the slider is formed of porous bronze and in which lubricant-containing wick material is disposed in the slider normal to and within the crankpin bearing hole.