A high pressure piston for use within a liner for a reciprocating pump comprises an elastomeric seal and a metallic annular bearing ring having relatively high heat conductivity compared to the liner and seal. During the pump's pressure stroke a portion of the bearing ring elastically expands radially to narrow or close the extrusion gap. Narrowing the gap decreases the tendency for extrusion of piston seal elastomeric material, while closing the gap tends to block elastomer extrusion and establish sliding contact between the bearing ring and the liner. Sliding contact between the bearing ring and liner prevents portions of the piston hub (e.g., a transverse flange) from contacting the liner and damaging it. The heat-conductive bearing ring improves scavenging of frictional heat near the extrusion gap, extending the service life of piston seal elastomer while reducing bearing ring and liner wear. Under reduced pressure on the pump's return stroke, the bearing ring elastically contracts radially, slightly widening the extrusion gap. On the pump's return stroke the slightly wider extrusion gap allows cooling water directed generally toward the proximal flange of the piston hub to better cleanse, as well as cool, the liner wall.