flat engine
engine configuration with horizontally opposed cylinders, usually a flat-V or boxer configuration
flat-four
horizontally opposed four-cylinder piston engine
flat-eight
horizontally-opposed eight cylinder piston engine
ABC Gnat
O-2 piston aircraft engine
flat-six engine
horizontally opposed 6-cylinder piston engine
Continental Tiara series
air-cooled, horizontally-opposed piston aircraft engine family