A monobloc piston has at least two steel parts (13, 15) welded together to define an inner cooling gallery (32). An outer ring belt (14) is spaced from an inner annular support wall (34) and is joined by a combustion bowl (28) and a lower wall. A pair of pin bosses (38) have axially aligned pin bores (44). A skirt (46) is formed as one immovable piece with the pin bores. The piston has the following dimensional relationships: ISMD = 42-55% of BD, where ISMD is a mean diameter on the inner support wall and BD is an outer diameter of the ring belt wall, ISW = 3-8% of BD, where ISW is a sectional width of inner wall; CH 53% of BD where CH is a compression height measured between the pin bore axis and the upper surface, TLH 4% of BD, where TLH is a top land height measured between the top of the upper ring groove and the upper surface, SL = 30-80% of BD, where SL is a length of the skirt measured between the upper and lower ends of the skirt, SW = 2.5 - 6.5% of BD, where SW is a thickness of the skirt, and GV = 150-250% of BD2 and 5-20% of BD2 x CH, where GV is a volume of the oil gallery.
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