Franklin, with Continental and Lycoming, was one of the "big three" of horizontally-opposed engines in the late 1930s and early 1940s.  Franklin became an "also ran" after World War II.

History

1893: H.H. Franklin Manufacturing is formed.

1902: The first Franklin automobile is sold.  It has a four cylinder air-cooled engine.

1934: The Depression bankrupts Franklin

1937: A group of Franklin engineers buy the assets of H.H. Franklin Co. and form Air Cooled Motors Development Co.  Engines are marketed under the Franklin name.

1945: Air Cooled Motors is acquired by Republic Aircraft to produce engines for its RC-3 Seabee.

1948: Tucker Industries buys Air Cooled Motors from Republic to develop a liquid-cooled engine for the 1948 Tucker.

1961: Tucker sells Air Cooled Motors to Aero Industries which renames the company Franklin Engine Co.

1975: Rights to Franklin engines are bought by the Polish government which begins production of PZL-Franklin engines in Rzeszow.

Franklin 6AC-298/ O-300

In 1941, Franklin introduced four new engines based on a 4.25 x 3.5 inch cylinder.  These engines were the 4-cylinder 4AC-199, the 6-cylinder 6AC-298, the 8-cylinder 8AC-398 and the 12-cylinder 12AC-596.

The U.S. military gave the 6AC-298 the designation O-300.  Production ended in 1945.  A later Continental engine received the same designation.

6-cylinder, horizontally-opposed, air-cooled piston engine

Displacement: 298 cubic inches (4.9 litres)

Bore x stroke: 4.25 x 3.5 inches