An oilless reciprocating compressor for use in a refrigerated controlled atmosphere has an uncooled, fully sealed crankcase. A bypass tubing connects the interior of the crankcase with the inlet of the cylinders, so that blow-by is returned from the crankcase to the inlet of the cylinder for recirculation. In consequence no blow-by gas escapes into the controlled atmosphere. The device includes standard pistons, and has an improved cylinder head seal utilizing o-rings.