Patent ID: 8141476

Claim:
A durable piston comprising a piston head having an annular wall; and a ring unit mount being annular, being formed around the annular wall of the piston head and having a distal edge; a proximal edge; a distal ring groove being formed transversely in and around the ring unit mount adjacent to the distal edge and having an inner wall; a proximal ring groove being formed transversely in and around the ring unit mount adjacent to the proximal edge, being adjacent to and being shallower than the distal ring groove and having an inner wall; a distal positioning protrusion being formed on and protruding towards the distal ring groove from the distal edge of the ring unit mount; and a proximal positioning protrusion being formed on and protruding towards the proximal ring groove from the proximal edge of the ring unit mount; and a durable ring unit being mounted around the ring unit mount and having a distal ring being mounted around the distal ring groove and having a distal surface; a proximal surface; an outer edge; an inner edge being separated from the inner wall of the distal ring groove; and a gap being formed in the distal ring, forming two separated ends of the distal ring and corresponding to the distal positioning protrusion; a shoulder being annular and being formed on and protruding transversely from the outer edge of the distal ring adjacent to the proximal surface of the distal ring; and a proximal ring being mounted around the proximal ring groove and having an outer edge; an inner edge being separated from the inner wall of the proximal ring groove; a distal surface abutting the proximal surface of the distal ring and having an oblique portion being formed on the distal surface of the proximal ring adjacent to the inner edge of the proximal ring and being angled inward, toward the proximal edge of the ring unit mount from the outer edge toward the inner edge; a proximal surface having an oblique portion being formed on the proximal surface adjacent to the outer edge of the proximal ring and being angled toward the distal surface of the proximal ring from the inner edge toward the outer edge; and a gap being formed in the proximal ring, is staggered with the gap of the distal ring, forming overlapping first and second ends of the proximal ring and corresponding to the proximal positioning protrusion.