Franklin 6A4

O-335/6AC
Type	Flat-6
National origin	United States
Manufacturer	Franklin Engine Company
Major applications	Bell 47

The Franklin O-335 (Company designations variations on 6A4) was a series of air-cooled aircraft engines of flat-six configuration.

These engines were commonly vertically mounted and used to power many early helicopters in the United States. They were closely related to the 2A4 and 4A4 2- and 4-cylinder engines. In various subtypes, the 6A4 remained in continuous production from 1945 to the time Franklin's United States operations ceased in 1975, with versions continuing in Polish production into the 1990s.

In 1947 this engine was modified into a water-cooled version by the Tucker Car Corporation for use in the 1948 Tucker Sedan. Tucker liked the engine so much that he purchased the Aircooled Motors/Franklin Engine Company, and it remained under the ownership of the Tucker family until 1961.

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