Abstract: A bearing assembly for a piston rod of a linearly reciprocating piston type engine having a cylinder (11), a piston (12) movable axially in cylinder (11), at least one piston rod (13) secured to and projecting axially from one side of the piston to be received in a respective bearing assembly (17) mounted in a cover assembly at one end of the cylinder and which guides the movement of the piston/piston rod assembly, said bearing assembly comprising: an inner annular array of balls (18) which roll on the cylindrical outer surface of the piston rod (13) during its reciprocation, and housed in an annular space defined between the cylindrical outer surface of rod (13) and a sleeve (19); bearings (20) which rotatably mount sleeve (19); and means for rotating the sleeve (19) about the axis of the piston rod (13) so as to apply continuous rotation to the balls during each cycle of reciprocation of the piston rod and thereby enable a hydrodynamic lubrication of the bearing assembly.