In the U.S., almost the only proponent of the air-cooled engine during World War I was the Lawrance Aero Engine Company. This small New York City firm had produced the crude opposed twins that powered the Penguin trainers, which were supposed to be the stepping-stone to the Jenny for aspiring military pilots. The Penguins were not intended to fly but apparently could taxi at a speed that would provide some excitement for trainees as they tried to maintain directional control and develop some feel of what flight controls were all about. The Lawrance twins, which can be seen in many museums, had directly opposed air-cooled cylinders and a crankshaft with a single crankpin to which both connecting rods were attached. This arrangement resulted in an engine that shook violently at all speeds and was therefore essentially useless for normal powered flight. After World War I, some attempts were made, generally unsuccessful, to convert the Lawrance twins into usable engines for light aircraft by fitting a two-throw crankshaft and welding an offset section into the connecting rods. This proves that the desire to fly can be very strong indeed in some individuals.

After the end of World War I, the Lawrance engineers worked with both the Army and the Navy in developing a nine-cylinder radial engine, the Model  J-1. It was the best American air-cooled engine at the time and passed its 50-hour test in 1922. A unique feature of the engine was the use of exhaust valves with hollow stems that were partly filled with mercury as a means of carrying heat from the head of the valve through the stem to the valve guide. Heron and Gibson had experimented with hollow stemmed valves for a number of years using water or mercury as heat transfer agents. The use of mercury was not really satisfactory since it would not wet the surface of the inside of the valve stem and therefore did not transfer heat well. Water had been found to be wholly unsuitable.

The U.S. Navy had decided to build aircraft carriers and since it badly needed light, reliable engines it gave a contract to Lawrance for the J-1 radial and ceased buying the liquid-cooled Hispano-Suiza engines manufactured by Wright under license as a means of pressuring Wright and other companies into developing radial engines. The Wright Aeronautical Corporation bought the Lawrance Company, largely at the urging of the Army and Navy, and the later engines were known as Wrights. These engines, known as the J series, were dominant in the 1920s and the J-5, which was the first American production engine to use salt-cooled exhaust valves, achieved everlasting fame as the powerplant that carried Charles Lindbergh across the Atlantic.

The power output of other engines that were developed during the World War II period was also increased substantially. This can be attributed largely to advances and innovations in cylinder design and construction, metallurgical progress, and improvements in foundry capability in the U.S. American foundry men, through intensive experimentation, had developed procedures for casting thin, closely spaced fins on a production basis and had progressed well beyond the British in this critical area. One cannot help but be impressed, in looking at a World War II era Pratt & Whitney or Wright engine, at the skill of those who produced those beautiful cylinder head castings literally by the hundreds of thousands, and in some cases, by the millions.

By the early 1930s, cylinder design in the U.S. for large radial engines had become somewhat standardized, with bores of about six inches and two large valves per cylinder in a hemispherical combustion chamber. Valve actuation was by means of ring cams housed in the nose section or the crankcase, enclosed push rods, and rocker arms in individual housings cast integrally with the cylinder head. While all engines of any consequence had enclosed valve mechanisms, it wasn't until 1932 that automatic pressure lubrication with engine oil was introduced on the Pratt & Whitney Wasp. Much of the subsequent development work involved experimentation with materials, sodium-cooled valves, improvements in piston rings, better foundry practices, and many other minor improvements that would extend engine life and allow higher power outputs. Near the end of World War II the much more widespread use of forged cylinder heads with machined fins was a major factor in allowing engines to be operated at higher manifold pressures because of the greater mechanical strength and better heat dissipation capabilities of such heads.

Engine development has always been a long, often unrewarding, and always expensive process that can often be affected by events beyond the designer's control, such as limited finances or a major war. One of the inescapable conditions imposed by war is that survival and victory often depend on the ability to adapt and change rapidly to meet new challenges. In the late 1930s and throughout World War II military necessity made performance the dominant concern, and when cost became a secondary factor progress was almost inevitably more rapid. The more rigorous operating conditions revealed weaknesses in cylinders and many other engine parts that might have remained hidden in normal commercial use, and by the end of the war the air-cooled cylinder had very nearly reached its peak in both performance and durability. Other than direct fuel injection, which came into relatively limited use in the last years of World War II and allowed much more accurate fuel-air mixture distribution, no major innovations appeared since it was becoming increasingly evident that large radial engines were no longer of major importance in commercial or military aviation. The gas turbine had arrived and a new era had begun.