Under a state in which the combination oil ring is mounted in an oil ring groove before inserted into a cylinder bore, when an inclined surface of an upper ear portion of an expander spacer is held in contact with an inner peripheral surface of an upper segment and an inclined surface of a lower ear portion of the expander spacer is held in contact with an inner peripheral surface of a lower segment, an upper protrusion amount (P1) from an outer peripheral-side end surface of the expander spacer to an outer peripheral surface vertex of the upper segment is larger than a lower protrusion amount (P2) from the outer peripheral-side end surface of the expander spacer to an outer peripheral surface vertex of the lower segment. When the combination oil ring is inserted into the cylinder bore, the sealing performance can be improved to reduce the oil consumption.

A piston is provided with a gapless ring positioned in a subport position. Specifically, the gapless ring is positioned between the air ports and the crankcase of the engine. The gapless ring includes a rail and an annular ring received within the rail. The ring includes a ring gap and the rail includes a rail gap. When the rail receives the annular ring, the ring gap and rail gap are spaced apart in a circumferential direction such that the gaps do not overlap. In certain embodiments, the rail includes a notch. When the annular ring is received within the notch, the annular ring and rail are biased against rotating with respect to each other. In certain other embodiments, the gapless ring includes an oil scraping surface.

A piston is provided with a gapless ring positioned in a subport position. Specifically, the gapless ring is positioned between the air ports and the crankcase of the engine. The gapless ring includes a rail and an annular ring received within the rail. The ring includes a ring gap and the rail includes a rail gap. When the rail receives the annular ring, the ring gap and rail gap are spaced apart in a circumferential direction such that the gaps do not overlap. In certain embodiments, the rail includes a notch. When the annular ring is received within the notch, the annular ring and rail are biased against rotating with respect to each other. In certain other embodiments, the gapless ring includes an oil scraping surface.

A sliding member includes a base material, and an amorphous hard carbon film formed on a surface of the base material, in which a sp.sup.2 ratio of the amorphous hard carbon film increases from an inner surface side corresponding to the base material side toward an outer surface side thereof to become a maximum value and further decreases toward the outer surface side.

A play-free oil scraper ring has a cross section with a running surface leg having at least one scraper web that is situated on a running surface, an upper piston ring flank leg that is connected to the running surface leg, and the upper surface of which forms an upper piston ring flank. A lower piston ring flank leg is likewise connected to the running surface leg, the lower surface of which forms a lower piston ring flank. The upper and/or the lower piston ring flank leg are/is elastic, taper(s) inwardly in the radial direction, and their/its piston ring flank(s) essentially form(s) a conical surface in an uninstalled state. The uninstalled piston ring has an axial height that exceeds a width of a piston ring groove for which the piston ring is intended.

A sliding member includes a base material and an amorphous hard carbon film formed on a surface of the base material, and the amorphous hard carbon film has a graded structure in which a sp.sup.2 ratio increases from an inner surface side corresponding to the base material side toward an outer surface side. A piston ring includes the sliding member.

In a sliding structure for an internal combustion, a cylinder has recesses in a stroke center region. Piston rings have inclined surfaces on an outer circumferential surface, and a lubricating oil flows between the inner wall surface and the outer circumferential surface that relatively move via the inclined surfaces. At any RPM equal to or greater than at idle, a center friction coefficient at the stroke center region through which the piston rings pass at the highest speed is less than a center friction coefficient when no recesses are formed in the stroke center region. Contrarily, at the RPM, an outside friction coefficient when the piston rings pass through a region outside the stroke center region is less than an outside friction coefficient when the recesses are formed in the outside region. As a result, further improved low fuel efficiency is achieved for the dimple liner technique.

A piston ring (10) has a running surface (12) and a flank surface (14) which are coated. The uppermost layer of the running surface (12) is a hydrogen-containing or a hydrogen-free DLC layer, and the uppermost layer of at least one flank surface (14) is a chromium layer. A method of producing a piston ring (10) includes forming a DLC layer as the uppermost layer of the running surface (12), and forming a chromium layer as the uppermost layer of at least one flank surface (14).

To provide a sliding member, such as a piston ring for an internal combustion engine, having low friction and excellent toughness. The above-described problem is solved by a sliding member (10) such as a piston ring coated with a CrBTiV(Mn, Mo)-N-based alloy film (2) on a sliding surface (11) thereof, and configured so that the alloy film (2) contains one or both of Mn and Mo and has a total content of the Mn and the Mo within a range of 2 mass % or less. Preferably, a B content is within a range of 0.1 mass % to 1.5 mass %, inclusive, a V content is within a range of 0.05 mass % to 1 mass %, inclusive, and a Ti content is within a range of 0.05 mass % to 1.5 mass %, inclusive.

A piston for an internal combustion engine may include a piston head, an encircling fire land, and at least one annular groove structured to receive a piston ring and disposed contiguous to the fire land. The piston may include a first fire land portion having a first axial height h1, a second fire land portion having a second axial height h2, and a third fire land portion having a third axial height h3. The first fire land portion may have an upper external diameter D1. The third fire land portion may have a lower external diameter D3. A transition from the first to the second fire land portion may have a first external diameter D12. A transition from the second to the third fire land portion may have a second external diameter D23. The relationship (D12D23)/h2>(D23D3)/h3 may apply.