| 摘要 |
PROBLEM TO BE SOLVED: To prevent exhaust gas from deteriorating and reduce the lubricating oil consumption both resulting from lubricating oil blowing-up into combustion chambers, and improve the maintenance of gastightness of each piston-ring, in gastight devices with piston rings that are fitted into at least two upper and lower stepped ring grooves in the periphery of pistons reciprocating in cylinders and are so shaped that the cut gap of the lower piston ring is larger than that of the upper piston ring. SOLUTION: A lower piston ring 6 has, on its cut 6a, protrusions 7 constricting a gap dimension P2 to a narrower one R2 on the circumference. A gap dimension S2 between a peripheral portion 2b of a piston 2 below the lower piston ring 6 and an interior surface 1a of a cylinder 1 is made larger than a gap dimension S1 between a peripheral portion 2a of the piston 2 above the lower piston ring 6 and the interior surface 1a of the cylinder 1.
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