PURPOSE:To prevent the movement of split parts of piston ring due to compressor vibration during operation and the formation of blow holes by pre-forming the expansion ring into the internal configuration of a worn piston ring. CONSTITUTION:A frame ring 4, designed to expand a piston ring 1, is formed into a circle having a radius e extending from its center 0, with the circle alighning with that of the piston ring 1 at points 0a-0c, spaced at equal angle and interval in the center of partial components of the ring 1. The frame ring is formed into the circle by connecting circular arcs having radius r, with centers of these components as its center. Piston ring 1 is composed of the partial components 1a through 1c with its internal circumference consisting of circular arcs provided with radii, ra through rc and having 0a through 0c, spaced at equal distance from the center 01 as its center. As the piston ring 1 is engaged, in its internal circumference, by the circular arc of the frame ring 4, mutual circumferential movement of the piston ring 1, when worn out, can be prevented.
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