A steel piston for heavy-duty diesel engines designed with features that increase adiabaticity, reduce weight, and improve passive cooling without loss of power density is provided. The piston includes a pair of skirt walls extending longitudinally from a crown to a lower end. The skirt walls include a plurality of heat sink wells extending axially from the crown to a lower end of the skirt wall. Each heat sink well is at least partially filled with a cooling medium and is sealed. The piston also includes a plurality of ribs each radially aligned with one of the heat sink wells and extending inwardly and upwardly from a first rib end at an inner surface of the skirt wall to a second rib end at the inner surface of the crown.