Patent ID: 7870819

Claim:
A piston device, disposed within a cylinder having an inner wall, said piston device comprising: a piston body, having at least one annular groove on the side wall of said piston body; and at least one piston ring, made of flexible material, and said piston ring mounted in said annular groove, and the outer diameter of said piston ring mounted on in said annular groove is larger than the diameter of said piston body; wherein said piston ring contacts with said inner wall but said piston body has no contact with said inner wall while said piston device moves reciprocally at the axial direction of said cylinder; wherein said piston device matches the following formula: A±E=B±G+ 2* D C±F=A±E− 0.1 wherein A represents the inner diameter of said cylinder, and B represents the diameter of said annular groove, and C represents the diameter of said piston body, and D represents the thickness of said piston ring, and E, F, G represent the tolerance of A, C, B respectively; wherein said B is 27.89-27.91 mm, said E is 0.05 mm, said F is 0.05 mm, and said G is 0.01 mm.