Lycoming produced a four-cylinder horizontally opposed engine in 1938 in addition to its radial engines. Introduced in 1942, the O-435 was a six-cylinder version of the four-cylinder O-290. These engines were also built as tank engines during the WW2 (Locust tank) and Korean War eras. Other models were fitted to helicopters (VO-435), or the turbocharged (TVO-435), as well as geared (GO-435) and geared, supercharged versions (GSO-435).
One of the Museum's examples of this engine is mounted on a test stand as used by engine overhaul companies to ground run an engine. The Museum's Bell 47J, that is awaiting restoration, is powered by a version of this engine modified for helicopters. An engine on display shows the vertical mounting structure and turbosupercharger of the TVO-435 model.