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: An engine piston has a crown portion generated about a longitudinal piston axis and, extending axially form the crown, a pair of gudgeon pin bosses defining a pin axis and spaced apart at opposite sides of an orthogonal thrust axis. A circumferentially discontinuous skirt defines lateral thrust and non-thrust bearing regions, respectively, centered on the thrust axis, each joined by convexly bowed reinforcement ribs to the bosses. The bearing region on the non-thrust side of the piston is circumferentially shorter than thrust region, reducing piston mass to one side of the piston, and the reinforcement ribs are offset in pin axis direction where they intersect the bosses. Mass reducing recesses are formed in the underside of the crown between the unskirted periphery reinforcement ribs and bosses, the offset ribs permitting the recesses to extend further and to permit greater reduction in mass.

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.

Abstract: An engine piston (10) has a crown portion (12) generated about a longitudinal piston axis (14) and, extending axially from the crown, a pair of gudgeon pin bosses (26, 28) defining a pin axis (30) and spaced apart at opposite sides of an orthogonal thrust axis (40). A circumferentially discontinuous skirt defines lateral thrust and non-thrust bearing regions (34, 36), respectively, centered on the thrust axis, each joined by convexly bowed reinforcement ribs (62, 64, 66, 68) to the bosses. The bearing region (36) on the nun-thrust side of the piston is circumferentially shorter than thrust region (34), reducing piston mass to one side of the piston, and the reinforcement ribs are offset in pin axis direction where thy intersect the bosses. Mass reducing recesses (106, 108) are formed in the underside of the crown between the unskirted periphery (37), reinforcement ribs and bosses, the offset ribs (66, 68) permitting the recesses to extend further as at (110) and permit greater reduction in mass.