Abstract: In a reciprocating piston engine, the piston has a top ring and a second ring received in respective ring grooves. The top ring has two end surfaces defining therebetween a gap the open area of which on the side of the combustion chamber, when the top ring abuts against the upper surface of the ring groove, is smaller than an open area of the gap facing a second land clearance when the top ring seats on the lower surface of the ring groove. As the top ring seats on the lower surface of the ring groove during the power stroke when the engine is operated at a high-speed, high-loaded condition, the greater open area of the gap rapidly introduces the combustion pressure into the second land clearance to thereby reduce oil consumption. When the engine works at a low-speed, low-loaded condition, the top ring abuts against the upper surface of the ring groove during the intake and compression strokes, where the smaller open area of the gap reduces the flow of oil from the crankcase to the combustion chamber.