Patent ID: 7997185

Claim:
A piston, comprising: a piston body having an outer circumferentially disposed groove, a surface of said piston groove providing a first seat; a first ring selectively disposed within said piston groove and including a surface providing a second seat; a blocking ring with a generally annular body, an inner end portion, and an outer end portion, said inner end portion of said blocking ring selectively engaging said first seat, said outer end portion of said blocking ring selectively engaging said second seat, said blocking ring being selectively subjected to an axial force to controllably limit outward radial excursion of said first ring; and at least one vent passage formed in said piston body, said vent passage leading from a first volume to a second volume, said first volume bounded at least by an inner surface of said first ring and said piston groove, said second volume bounded by said first ring, a cylinder bore and a second ring; wherein an upper surface of said outer groove includes a discontinuity, thereby spacing an upper surface of said blocking ring away from at least a portion of said upper surface of said upper groove.