In all, by the war’s end, five different designs of cylinder and head, plus a series of five sets of differently designed cooling baffles had been tried (two of those baffle types were installed by the hundreds in hectic field-update programs half a world away). And continuing lack of engine durability was a strong factor in a March 1945 change of B-29 tactics, bringing the aircraft down from their originally designed bombing altitude of 30,000 feet to just 5,000 to 8,000 feet at night. Why? Low air density above 20,000 feet resulted in chronically poor cooling, even with the advanced “W” fin barrel-cooling system, which crowded 54 fins onto each cylinder. Instead of being machined from solid, W fins were pressed from thin aluminum sheet as 180-degree elements which were then swedged by special tooling into shallow grooves machined into barrel outer diameters. As the war ended in August, a program to fan cool the R-3350 engine was dropped.