In accordance with the invention, an internal combustion engine having reciprocating piston sleeves is realized comprising an engine block with a pair of cylinders, each cylinder having an intake port, an exhaust port and two linearly opposing pistons connected to two opposing crankshafts. A pair of piston sleeves are reciprocatingly mounted in each cylinder, one piston sleeve around each piston. Each piston sleeve is connected to one of two eccentric shafts that run parallel and adjacent to each crankshaft. The piston sleeves have ported slots in communication with either the intake ports or the exhaust ports of each cylinder. The eccentric shafts are mechanically connected to the crankshafts such that they move in unison.