The Curtiss H-1640 Chieftain was an unusual American 12-cylinder radial aero engine designed and built by the Curtiss Aeroplane and Motor Company in the mid-1920s.

The H-1640 was an air-cooled 12 cylinder two-row radial with the cylinder rows aligned rather than staggered as in most multi-row radials.

One piece cylinder heads shared a single overhead camshaft and the propeller was directly driven.

The H-1640 was the first airworthy 'inline radial' and was sponsored for flight testing in a range of aircraft by the U.S. Government.

Cooling problems with the rear cylinders caused the project to be canceled with few production engines being built.