In the late 1920s, the Lycoming Manufacturing Corporation of Williamsport (Lycoming County), Pennsylvania entered the aircraft engine business. At the time, Lycoming was a major supplier of automobile engines to a variety of different manufacturers. Lycoming quickly found success with a reliable nine-cylinder radial of 215 hp (160 kW), the R-680. However, the company wanted to expand into the high-power aircraft engine field.

In 1932, Lycoming became aware of the Army Air Corps’ (AAC) program to develop a high-performance (Hyper) cylinder that would produce one horsepower per cubic inch displacement and enable a complete aircraft engine to produce one horsepower per pound of weight. The AAC had contracted Continental Motors in 1932 to work with the Power Plant Branch at Wright Field, Ohio on developing an engine utilizing Hyper cylinders. The engine type was set by the AAC as a 1,200 hp (895 kW), flat, liquid-cooled, 12-cylinder engine that utilized individual-cylinder construction. The flat, or horizontally-opposed, engine configuration was selected to enable the engine’s installation buried in an aircraft’s wings.

Lycoming saw an opportunity to quickly establish itself as a high-power aircraft engine manufacturer by creating an engine that would satisfy the AAC’s requirements. On its own initiative, Lycoming began work on its own Hyper cylinder with the intent of developing a 12-cylinder engine. Lycoming chief engineer Val Cronstedt was put in charge of the project, and he was assisted by Samuel Hoffman, Clarence Wiegman, and other engineers. The AAC encouraged Lycoming’s involvement and provided developmental support, but the AAC did not initially provide financial support.

Lycoming started serious developmental work on the new engine in 1933. Various single-cylinder test engines were built and tested in 1934. Also in 1934, Hoffman and Wiegman filed for a number of patents that detailed some proposed aspects of the complete 12-cylinder engine. In 1935, the AAC became more interested in the engine and began supporting Lycoming’s efforts. Single-cylinder testing yielded positive results, with the engine passing a 50-hour test in May 1936 and producing 228.7 hp (170.5 kW) at 3,000 rpm from its 102.8 cu in (1.69 L) displacement during maximum performance tests in July 1936. That same year, the AAC contracted Lycoming to build a complete engine. Lycoming had spent $500,000 of its own money and had finalized the design of its engine, which was designated O-1230 (also as XO-1230). Construction of the first O-1230 was completed in 1937, and the engine was ready for endurance testing in December of that year.

The Lycoming O-1230 had a two-piece aluminum crankcase that was split vertically. Six individual cylinders attached to each side of the crankcase. The cylinders were made of steel and were surrounded by a steel water jacket. Each cylinder had a hemispherical combustion chamber with one intake and one sodium-cooled exhaust valve. A cam box mounted to the top of each cylinder bank, and each cam box contained a single camshaft that was shaft-driven from the front of the engine.

Top: Drawing of the O-1230 from US patent 2,119,879 for a proposed fuel injection system that can be seen in the induction manifold. The drawing shows details of the engine’s construction including the split crankcase and overhead camshaft. Bottom: Intended for installation buried in an aircraft’s wing, the O-1230’s height was kept to a minimum. The long nose case would aid in streamlined wing installations. Note that the supercharger’s diameter was slightly in excess of the engine’s height.

A downdraft carburetor fed fuel into the single-speed, single-stage supercharger mounted at the rear of the engine. The supercharger’s 10 in (254 mm) diameter impeller was driven at 6.55 times crankshaft speed. It provided air to the intake manifold that sat atop the engine. Individual runners provided air to each cylinder from the intake manifold. Lycoming had experimented with direct fuel injection on test cylinders and US patent 2,119,879 was applied for in November 1934 that detailed a fuel injection system for the O-1230. In the patent, fuel injectors would be installed in the removable cover of the intake manifold with one injector spraying into each runner and toward the cylinder. However, it is unlikely that any O-1230 ever used fuel injection.

Exhaust was expelled out the lower side of the cylinders and collected in a common manifold for each cylinder bank. An extended nose case housed beveled planetary gears for the propeller’s gear reduction. The propeller turned at .40 crankshaft speed, with options for a .50 or .333 reduction. On the top of the O-1230, just behind the gear reduction, was the engine’s sole magneto. The magneto was connected to two distributors, each driven from the front of the camshaft drive.

The O-1230 with one style of engine mount that was secured between the camshaft housing and cylinders. Note the induction manifold and individual runners atop the engine.

The O-1230 had a 5.25 in (133 mm) bore and a 4.75 in (121 mm) stroke. The engine’s total displacement was 1,234 cu in (20.2 L), and it had a 6.5 to 1 compression ratio. The O-1230 produced 1,200 hp (895 kW) at 3,400 rpm for takeoff, 1,000 hp (746 kW) at 3,100 rpm for normal operation, and 700 hp (522 kW) at 2,650 rpm for cruise operation. The engine had an overspeed limit of 3,720 rpm for diving operations. The O-1230 was 106.7 in (2.71 m) long, 44.1 in (1.12 m) wide, and 37.9 in (.96 m) tall. The engine weighed 1,325 lb (601 kg).

After completing a 50-hour type test in March 1939, the O-1230 was rated at 1,000 hp (746 kW). Continued development pushed the engine’s rating up to 1,200 hp (895 kW). The O-1230 was installed in a Vultee YA-19 attack aircraft that had been modified as an engine testbed and redesignated XA-19A (38-555). Some sources list the designation as YA-19A, but “Y” was typically used for pre-production aircraft, while “X” was for experimental aircraft. The O-1230-powered XA-19A first flew on 22 May 1940, the flight originating at Vultee Field in Downey, California. The aircraft and engine combination were transferred to Wright Field, Ohio in June 1940 and then to Lycoming on 27 March 1941. By this time, the AAC had already moved away from the buried-engine-installation concept and was interested in more powerful engines.

The XA-19A is seen with its Wright Field markings. The scoop above the cowling brought air into the engine’s carburetor. Louvered panels allowed heat generated by the exhaust manifold to escape the cowling. Note the large exhaust outlet. The radiator positioned under the engine added bulk to the O-1230’s installation. The aircraft’s tail was modified to compensate for the larger and longer nose needed to house the O-1230.

While the O-1230’s power output was on par with many of its contemporaries, such as the Allison V-1710, the O-1230 did not offer the same development potential or reliability as other engines. The O-1230 was cancelled in favor of other projects, and the engine was subsequently removed from the XA-19A airframe. The XA-19A was transferred to Pratt & Whitney on 8 August 1941, where an R-1830 was subsequently installed, and the aircraft was redesignated XA-19C.

Lycoming was still interested in developing a high-power engine and used O-1230 components to create the 24-cylinder XH-2470. In some regards, the Lycoming XH-2470 was two O-1230 engines mounted to a common crankcase. Lycoming started initial design work on the engine as early as 1938. An O-1230 is on display at the New England Air Museum in Windsor Locks, Connecticut.