The Lycoming Model O-145 engines are four cylinder, horizontally opposed, air-cooled aircraft engines.   The A, B, and C series are identical in design with only such changes in construction as to permit the different horsepower ratings.   The O-145 is direct drive while the GO-145 has a reduction gearing ratio of 27 to 17.    The piston displacement for all models is 144.5 cubic inches.   The A series is rated at 50-55 HP at 2300 RPM, the B series at 65 HP at 2550 RPM, and C series at 75 HP at 3100 RPM.   The G series is 75 HP at 3200 RPM with a propeller RPM of 2015.   The number following the letter denotes the equipment on the engine.   1 is single ignition, 2 is dual ignition, and 3 is dual ignition with generator and starter drives.