To give the new engine the ability to handle the heat of higher-power operation, head cooling was greatly boosted by switching material from cast-iron to aluminum, with three times iron’s heat conductivity. Because even high-strength aluminum is too soft to act as a valve seating, valves were seated on shrunk-in hard seat rings. Because of the higher heat expansion of the aluminum heads, it was decided to incorporate hydraulic clearance adjusters into the tops of the four pushrods. This was hoped to eliminate need for traditional manual adjustment of valve clearances, but particulates blocked ports in the hydraulic adjusters, sometimes causing collapse and valve noise. The cylinders remained cast-iron. New aluminum heads were cast from the same high-hot-strength alloys used in the engines that powered the B-17, P-47, and F4F Hellcat combat aircraft.