Abstract: A reciprocating piston gas compressor driven by an integral internal combustion engine is disclosed. The compressor may be used as an air compressor but has particular application for compressing natural gas from natural gas wells. It has few moving parts. The compressor has a reciprocating assembly, one end of which functions as the piston in an engine and the other end of which functions as the piston in a compressor. Motion of the assembly is regulated by a crankshaft. The crankshaft is connected to the reciprocating assembly by a linkage having arms which pass on either side of the assembly. The crankshaft preferably extends through a slot in the reciprocating assembly. Several compressors may be combined to make a multi-stage compressor.