The Franklin O-150 (company designation 4AC-150) was an American air-cooled aircraft engine of the late 1930s (first run in 1938). The engine is of four-cylinder, horizontally-opposed layout with a displaced of 150 cubic inches. The first model used a single magneto while later models had dual magnetos.  We have examples of both versions on display at Wings of History.

The engine was used on a number of light aircraft in the 1930s including:

Aeronca 50 Chief
Piper J-3 Cub
Rose Parakeet
Taylorcraft BF-50

This dual magneto engine was restored in 2019 to running condition by the students of instructor Paul Agaliotis at Gavilan College’s A&P school of Gilroy, California.

Check out the video of a test run

Engine Name	Franklin 4AC-150
Specifications
Manufactured by:	Aircooled Motors
Cylinder Configuration	opposed (flat)
Number of Cylinders	4
Bore (inches)	3.625
Stroke (inches)	3.625
Displacement (cubic inches)	149.6
Fuel Type	gasoline
Electrical	single magneto (early versions) or dual magneto (later versions)
Horsepower	60
Rated RPM	2400
Compression Ratio	6.66:1
Cooling System	air-cooled
Dry Weight (lb)	155
Fuel Consumption (gph)	4.4