A piston assembly for an internal combustion engine, comprising: a piston having a crown with an upper combustion surface and a lower wall surface with a cylindrical outer surface extending downwardly from said upper combustion surface and having a pair of pin bosses depending from said crown to provide a pair of pin bores having generally cylindrical bearing surfaces aligned along a pin bore axis with a top wall portion extending between said pin bosses, said top wall portion having a concave bearing surface forming a continuous concave bearing surface with said pin bores; a skirt constructed as a separate piece of material from said piston, said skirt being fixed to said piston against relative movement and having a cylindrical wall with inner and outer cylindrical surfaces; and wherein said piston further includes at least one rib extending upwardly from said top wall portion on opposite sides of said pin bore axis to said lower wall surface of said crown, said at least one rib joining said crown to said top wall portion provide structural support to said top wall portion against unwanted deflection, wherein said lower wall surface is an undercrown surface opposite said upper combustion surface and said piston further includes at least one laterally extending web extending between said pin bosses, said at least one rib extending upwardly from said at least one laterally extending web to said undercrown surface of said crown, wherein said at least one laterally extending web has a convexly contoured outer surface, characterized in that said pin bosses have generally cylindrical, convex outer surfaces forming a smooth generally cylindrical continuation of said convexly contoured outer surface of said at least one laterally extending web, and said inner surface of said skirt abuts said convexly contoured outer surface of said at least one laterally extending web and said outer surfaces of said pin bosses. a piston having a crown with an upper combustion surface and a lower wall surface with a cylindrical outer surface extending downwardly from said upper combustion surface and having a pair of pin bosses depending from said crown to provide a pair of pin bores having generally cylindrical bearing surfaces aligned along a pin bore axis with a top wall portion extending between said pin bosses, said top wall portion having a concave bearing surface forming a continuous concave bearing surface with said pin bores; a skirt constructed as a separate piece of material from said piston, said skirt being fixed to said piston against relative movement and having a cylindrical wall with inner and outer cylindrical surfaces; and wherein said piston further includes at least one rib extending upwardly from said top wall portion on opposite sides of said pin bore axis to said lower wall surface of said crown, said at least one rib joining said crown to said top wall portion provide structural support to said top wall portion against unwanted deflection, wherein said lower wall surface is an undercrown surface opposite said upper combustion surface and said piston further includes at least one laterally extending web extending between said pin bosses, said at least one rib extending upwardly from said at least one laterally extending web to said undercrown surface of said crown, wherein said at least one laterally extending web has a convexly contoured outer surface, characterized in that said pin bosses have generally cylindrical, convex outer surfaces forming a smooth generally cylindrical continuation of said convexly contoured outer surface of said at least one laterally extending web, and said inner surface of said skirt abuts said convexly contoured outer surface of said at least one laterally extending web and said outer surfaces of said pin bosses.