Three-piece type oil ring assembly

A three-piece type oil ring assembly adapted to be fitted in an oil ring groove of a piston disposed within a cylinder comprising:

• a spacer expander in the form of a gapped annulus made of section steel having substantially H-shaped radial cross section, and a pair of upper and lower side rails, each in the form of a gapped flat annulus,
• said spacer expander being adapted to be substantially held in between said upper and lower side rails,
• said spacer expander comprising an axially protruding outer portion disposed on an outer circumference thereof and facing an inner surface of said cylinder, an axially protruding inner portion disposed on an inner circumference thereof and facing an innermost wall of said oil ring groove, and an axially flat connecting portion extending between said outer and inner protruding portions, said outer and inner protruding portions and said connecting portion being axially symmetrically aligned, said outer protruding portion being axially wider than said flat portion, said inner protruding portion being axially wider than said outer protruding portion,
• said outer protruding portion having an upper side rail supporting surface and a lower side rail supporting surface respectively disposed on an upper end surface and a lower end surface thereof, said inner protruding portion having an upper side rail pressing surface and a lower side rail pressing surface respectively disposed on upper and lower halves of an outer circumferential surface thereof,
• said spacer expander having a plurality of radial slits formed therein so as to be alternately extending inwardly from an outer circumference toward an inner circumference and outwardly from said inner circumference toward said outer circumference of said spacer expander, and
• said outer protruding portion further having said side rail supporting surfaces formed so as to be respectively inclined toward the axially symmetrical centerline of the spacer expander along the radial direction from the outer circumference toward the inner circumference thereof at an acute angle with the radial direction,
• said inner protruding portion having said upper and lower side rail pressing surfaces formed respectively inclined outwardly along an axial direction from upper and lower ends thereof toward an intersection thereof with said connecting portion at an acute angle with the axial direction.