Cylinder rotor mounted in housing for rotation about a first axis of rotation, defining a central crankshaft chamber and having three pairs of cylinders located outwardly of the crankshaft chamber. Each cylinder being closed at a radially outer end by a cylinder cover affixed to the cylinder rotor. Its opposite sides of a plane that includes the cylinder axis and the first axis of rotation, at least one side wall of each said one chamber of each cylinder has overflow channels, and at least one side wall of each said other chamber of each cylinder has gas-exchange ports.