A number of changes were introduced into the Superfortress' production line in order to provide more cooling at low speeds, with the aircraft rushed into operational use in the Pacific in 1944. This proved unwise, as the early B-29 tactics of maximum weights, when combined with the high temperatures of the tropical airfields where B-29s were based, produced overheating problems that were not completely solved, and the engines having an additional tendency to swallow their own valves. Because of a high magnesium content in the potentially combustible crankcase alloy, the resulting engine fires — sometimes burning with as high a core temperature approaching 5,600 °F (3,100 °C)[1] from the Duplex Cyclone's magnesium engine crankcase alloys — were often so intense the main spar could burn through in seconds, resulting in catastrophic wing failure.[2]