The concept of an air gap insulated piston has been explored using bolted and welded/roll bonded designs. Pistons with bolted on crowns demonstrated the effectiveness of air gap insulation, but roll bonded and welded designs were found to be more robust and to provide the complete sealing of the air gap necessary for continued insulation. Evolution of the design to combine high insulation with adequate durability is discussed.Engine running times of up to 200 hours at full load have been achieved for an air gap piston which reduces heat flow to the crown by 33%. An improved design giving 41% reduction of heat flow has been tested for 78 hours at full engine load with no evident deterioration. Development is continuing to provide a fully durable piston achieving up to 50% reduction in heat flow.