Franklin 6 series

Franklin 6 series engines
Type	6-cyl. horizontally opposed air-cooled piston engine
National origin	United States
Manufacturer	Franklin Engine Company
Variants	Franklin O-335

The Franklin 6 is a series of air-cooled aircraft and helicopter engines of flat-6 configuration, produced from the 1930s to the 1970s, that found widespread use in helicopters and some light aircraft, (Military designation O-335, O-405 and O-425), as well as in the 1948 Tucker Sedan (with the engine modified for water cooling).

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Wikipedia