Piston rings have significant influences on the engine performance and tribology. The effect of such rings has a delicate relation with the axial motion in the groove. By simultaneously measuring the ring motion and the gas pressure between the rings in actual operations of automobile gasoline engines and diesel ones, the authors could observe undesirable motions none has ever expected. The motions gave the engines serious problems. One was the occurrence of ring scuffing when, in a gasoline engine operation, the top and second ring lifted off simultaneously before combustion T.D.C. The other was, in a supercharged diesel engine, the time which the top ring contacts on the upper groove surface is long, which resulted in an asymmetric temperature distribution in the ring and possibility of a great oil consumption.