An improved thin 3-piece oil ring 1 of an axial width not greater than 2 mm, is provided that has a corrugated spacer expander 2 of a circumferentially successive, substantially U-shaped configuration, and circular, upper and lower side rails 3, 4 holding the spacer expander 2 therebetween. The spacer expander 2 has circumferentially spaced outer crowns 5 and circumferentially spaced inner crowns 6; each of the outer crowns 5 being connected with next adjacent ones of the inner crowns 6 on opposite sides thereof by a pair of connecting members 11. The spacer expander 2 has both of the most-end portions thereof extended radially outwardly from the most-end one of the inner crowns 6 thereof forming abutting members 15, 16 for abutting each other at the point of force P. The outer crowns 5 at the upper and lower ends thereof have protrusions beyond the axial width of the outer crowns 5 forming upper and lower, outer supporting members 7, 8 bearing against the lower and upper sides of the upper and lower side rails 3, 4, at the outer portion thereof, respectively. The inner crowns 6 at the upper and lower ends thereof have extensions beyond the axial width of the inner crowns 6 forming upper and lower, inner supporting members 9, 10 bearing against the inner edges of the upper and lower side rails 3, 4, respectively. Each connecting member (11) has a radially elongated oil slot 13 made therethrough, and each of the abutting members 15, 16 having a radially elongated guide bar slot (14) made therethrough. The spacer expander 2 has a guide bar 12 inserted through the guide bar slots 14 and a plurality of the oil slots 13, at least one portion of the guide bar 12 being fixed to the spacer expander 2 by caulking. In one aspect of the present invention, the guide bar slots 14 have the axial width thereof made equal to the outer diameter of the guide bar 12. In another aspect of the present invention, the abutting members 15, 16 are such that the point of force P is positioned radially outwardly apart from the inner circumferential surface of the spacer expander 2 by a distance not less than 40% and not more than 60% of the radial width of the spacer expander 2. In a further aspect of the present invention, both of the most-end portions of the abutting members 15, 16 that extend farther radially outwardly beyond the point of force P, are apart from each other making an angle of not less than 5 degrees therebetween. With the arrangement described above, an improved 3-piece oil ring is obtained that has a significantly improved oil-sealing characteristics when used with a piston of internal combustion engines.
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