Piston

A piston, comprising:

• a base piston unit comprising a piston boss having a pin hole;
• a moving piston unit having a piston head and a circumferential wall part surrounding an entire circumference of the piston head and extending in reciprocation directions of the piston, holding the base piston unit in the circumferential wall part and being able to reciprocate with respect to the base piston unit;
• and
• an elastic member provided between the base piston unit and the moving piston unit and configured to expand and contract as the moving piston unit reciprocates with respect to the base piston unit,
• wherein the circumferential wall part has a pair of skirt parts opposing each other across the piston boss and a pair of side-wall parts coupling the skirt parts and each having an outer circumferential surface concaved toward a central axis of the piston;
• the side-wall parts have a penetration part each, which penetrates the side-wall part and which extends in the reciprocation directions;
• the base piston unit has projections extending perpendicular to the central axis of the piston and inserted in the penetration parts, a base-side projection that extends in a direction parallel to the reciprocating direction, and a fitting groove provided inside the base-side projection and opened toward an inner wall of the piston head in the reciprocating direction opposing the piston boss;
• the moving piston unit has a moving-part side projection shaped like a circular pillar and extending in the circumferential direction opposing the piston head to engage into the fitting groove, enabling the same to slide;
• and
• the elastic member is a disc spring having a hole at its center where the base-side projection and the moving-part side projection pass therethrough, and is provided between the base-side projection and the circumferential wall part of the moving piston unit.