Abstract: A method of forming a low-friction, helically ridged skirt surface of an engine piston comprises cutting a continuous groove in a piston body rotating about axis so that adjacent groove turns intersect as ridges and carry lubricating oil which forms a film on the ridged surface. The groove is cut by way of a diamond tool tip traversing along the axis. Whereas in a known, standard finish, the ridges have sharp peaks that can rupture the oil film, the tool is here modified to include a planing tip fixed spatially with respect to the tip so that after formation each ridge is planed flat to provide a plateau between groove turns that exerts less film rupturing pressure against a cylinder wall while defining a uniformly flat outer surface for the piston. The cutting may vary the piston diameter toward the ends of the body to effect barreling in which the relationship between plateau width and groove depth vary.

Abstract: A method of forming a low-friction, helically ridged skirt surface of an engine piston 30 (FIG. 2(a)) comprises cutting a continuous groove (40) in a piston body rotating about axis (34) so that adjacent groove turns intersect as ridges (42) and carry lubricating oil which forms a film on the ridged surface. The groove (40) is cut by way of a diamond tool tip (60) traversing along the axis (34). Whereas in a known, standard finish the ridges (42) have sharp peaks that can rupture the oil film, the tool is here modified to include a planing tip (70) fixed spatially with respect to the tip (60) so that after formation each ridge is planed flat to provide a plateau (44) between groove turns that exerts less film rupturing pressure against a cylinder wall whilst defining a uniformly flat outer surface for the piston. The cuting may vary the piston diameter towards the ends of the body to effect barreling in which the relationship between plateau width and groove depth vary.