The piston and piston ring are used in a severe contact environment in engine durability tests, which causes severe wear to the piston ring groove, leading to significant development costs for countermeasures. Conventionally, in order to ensure functional feasibility through wear on the piston top ring groove (hereinafter “ring groove”), only functional evaluations through actual engine durability testing were performed, and there was an issue in determining the limit value for the actual amount of wear itself. Because of this, the mechanism that may cause wear on the ring groove was clarified through past research, but this resulted in judgment criteria with some leeway from the perspective of functional assurance. To establish judgment criteria, it was necessary to understand both functional effect from ring groove wear and the mechanism behind it. For this research, the functional effect from wear on the upper surface of the ring groove and the mechanism that may cause this were clarified by performing two types of experiments. First, oil consumption tests were performed by incorporating a piston with a worn upper surface into an actual engine to confirm the functional effect from ring groove wear. The results showed that oil consumption deterioration and blow-by gas backflow occurred. Next, to clarify this phenomenon, uneven wear was simulated and a piston with additional machining was incorporated into a single-cylinder engine to compare piston ring motion on the thrust and anti-thrust side. Based on the results, it was confirmed that the piston ring does not sit on the upper surface of the groove on the anti-thrust side and that the blow-by gas flows back. The functional limit value and the allowance margin were grasped thanks to understanding the functional effect and the mechanism behind it, and the judgment criteria for wear amount was clarified, which enabled optimal design.