The Lycoming O-320 is a large family of naturally aspirated, air-cooled, horizontally-opposed four-cylinder, direct-drive engines produced by Lycoming Engines. Introduced in 1953, it is commonly used on light aircraft such as the Cessna 172 and Piper Cherokee, and remains in production as of 2026. Different variants are rated for 150 or 160 horsepower (112 or 119 kilowatts).Gunston 1989, p.98.
Design and development The O-320 family of engines includes the carbureted O-320, the fuel-injected IO-320, the inverted mount, fuel-injected AIO-320 and the aerobatic, fuel-injected AEIO-320 series. The LIO-320 is a "left-handed" version with the crankshaft rotating in the opposite direction for use on twin-engined aircraft to eliminate the critical engine.
The first O-320 (with no suffix) was FAA certified on 28 July 1953 to CAR 13 effective 5 March 1952; this same engine was later re-designated, without change, as the O-320-A1A. The first IO-320 was certified on 10 April 1961, with the AIO-320 following on 23 June 1969 and the first aerobatic AEIO-320 on 12 April 1974. The LIO-320s were both certified on 28 August 1969.
The O-320 family of engines externally resembles the Lycoming O-235 and O-290 family from which they were derived. The O-320 shares the same stroke as the smaller engines, but produces more power with the bore increased to . The design uses hydraulic tappets and incorporates the provisions for a hydraulically controlled propeller installation as well. The controllable-pitch propeller models use a different crankshaft from those intended for fixed-pitch propellers.Christy, Joe: Engines for Homebuilt Aircraft & Ultralights, pages 77-80 TAB Books, 1983.
The O-320 uses a conventional wet sump system for lubrication. The main bearings, connecting rods, camshaft bearings, tappets and pushrods are all pressure lubricated, while the piston pins, cylinder walls and gears are all lubricated by spray. The oil system is pressurized by an accessory-drive mounted oil pump. A remotely mounted oil cooler is used, connected to the engine by flexible hoses.
The versions of the carbureted O-320, are approved for the use of 87 AKI automotive gasoline. Models with 9.0:1 compression ratio are not approved, such as the H2AD model. All other 0-320s are approved for 91 AKI. Airframe approval is also necessary to use automotive gasoline in any certified aircraft.
The factory retail price of the O-320 varies by model. In 2010 the retail price of an O-320-B1A purchased outright was USD$47,076
Variants
O-320 series
O-320 (No suffix) later redesignated O-320-A1A at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Provisions for a controllable-pitch propeller and 25-degree spark advance. O-320-A1B at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as A1A but with straight riser in oil sump and -32 carburetor. O-320-A2A at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as A1A but with fixed-pitch propeller. O-320-A2B at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as A2A but with straight riser in oil sump and -32 carburetor. O-320-A2C at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as A2B but with retard breaker magnetos. O-320-A2D at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as E3D but with conical mounts. O-320-A3A at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as A1A but with 7/16" prop bolts. O-320-A3B at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as A3A but with straight riser in oil sump and -32 carburetor. O-320-A3C at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as A3B but with retard breaker magnetos.
O-320-B1A at 2,700 rpm, Minimum fuel grade 100/130 or 91/96 avgas, compression ratio 8.50:1. Same as A1A but with high compression pistons. O-320-B1B at 2,700 rpm, Minimum fuel grade 100/130 or 91/96 avgas, compression ratio 8.50:1. Same as B1A but with straight riser in oil sump and -32 carburetor. O-320-B2A at 2,700 rpm, Minimum fuel grade 100/130 or 91/96 avgas, compression ratio 8.50:1. Same as B1A but with fixed-pitch propeller provisions. O-320-B2B at 2,700 rpm, Minimum fuel grade 100/130 or 91/96 avgas, compression ratio 8.50:1. Same as B2A but with straight riser in oil sump and -32 carburetor.
O-320-B2C at 2,700 rpm, Minimum fuel grade 100/130 or 91/96 avgas, compression ratio 8.50:1. Same as B2B but with retard breaker magnetos. O-320-B2D at 2,700 rpm, Minimum fuel grade 91/96 avgas, compression ratio 8.50:1. Same as D1D but with conical engine mounts and no propeller governor. O-320-B2E at 2,700 rpm, Minimum fuel grade 91/96 avgas, compression ratio 8.50:1. Same as B2B except the carburetor is in the same location as the O-320-D models. O-320-B3A at 2,700 rpm, Minimum fuel grade 100/130 or 91/96 avgas, compression ratio 8.50:1. Same as B1A but with 7/16 inch propeller bolts. O-320-B3B at 2,700 rpm, Minimum fuel grade 100/130 or 91/96 avgas, compression ratio 8.50:1. Same as B1A but with 7/16 inch propeller bolts, a straight riser in oil sump, and -32 carburetor. O-320-B3C at 2,700 rpm, Minimum fuel grade 100/130 or 91/96 avgas, compression ratio 8.50:1. Same as B3B but with retard breaker magnetos.
O-320-C1A at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Low compression version converted through field conversion of B1A. O-320-C1B at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Low compression version converted through field conversion of B1B. O-320-C2A at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Low compression version converted through field conversion of B2A. O-320-C2B at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Low compression version converted through field conversion of B2B. O-320-C2C at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Low compression version converted through field conversion of B2C. O-320-C3A at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Low compression version converted through field conversion of B3A. O-320-C3B at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Low compression version converted through field conversion of B3B. O-320-C3C at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Low compression version converted through field conversion of B3C.
O-320-D1A at 2,700 rpm, Minimum fuel grade 100/130 or 91/96 avgas, compression ratio 8.50:1. Same as B3B but with Type 1 dynafocal mounts. O-320-D1B at 2,700 rpm, Minimum fuel grade 100/130 or 91/96 avgas, compression ratio 8.50:1. Same as D1A but with retard breaker magnetos. O-320-D1C at 2,700 rpm, Minimum fuel grade 100/130 or 91/96 avgas, compression ratio 8.50:1. Same as D2C but with provisions for a controllable propeller, O-320-D1D at 2,700 rpm, Minimum fuel grade 91/96 avgas, compression ratio 8.50:1. Same as the D1A but with Slick instead of Bendix magnetos and a horizontal carburetor and induction housing. This model was used in the Gulfstream American GA-7 Cougar twin. O-320-D1F at 2,700 rpm, Minimum fuel grade 91/96 avgas, compression ratio 8.50:1. Same as E1F except with high compression pistons.
O-320-D2A at 2,700 rpm, Minimum fuel grade 100/130 or 91/96 avgas, compression ratio 8.50:1. Same as D1A but with fixed-pitch propeller provisions and 3/8 inch attaching bolts. Used in the Symphony SA-160. O-320-D2B at 2,700 rpm, Minimum fuel grade 100/130 or 91/96 avgas, compression ratio 8.50:1. Same as D2A but retard breaker magnetos. O-320-D2C at 2,700 rpm, Minimum fuel grade 100/130 or 91/96 avgas, compression ratio 8.50:1. Same as D2A except -1200 series magnetos. O-320-D2F at 2,700 rpm, Minimum fuel grade 91/96 avgas, compression ratio 8.50:1. Same as E2F except with high compression pistons. O-320-D2G at 2,700 rpm, Minimum fuel grade 91/96 avgas, compression ratio 8.50:1. Same as the D2A except with Slick instead of Bendix magnetos and 7/16 inch instead of 3/8 inch propeller flange bolts. O-320-D2H at 2,700 rpm, Minimum fuel grade 91/96 avgas, compression ratio 8.50:1. Same as the D2G except with a O-320-B sump and intake pipes and with provisions for AC type fuel pump. O-320-D2J at 2,700 rpm, Minimum fuel grade 91/96 avgas, compression ratio 8.50:1.Similar to the D2G but with two Slick impulse coupling magnetos and the propeller governor pad, fuel pump and governor pads on the accessory housing all not machined. Used in the Cessna 172P. O-320-D3G at 2,700 rpm, Minimum fuel grade 91/96 avgas, compression ratio 8.50:1. Same as the D2G but with 3/8 inch propeller attaching bolts.
O-320-E1A at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as A3B but with Type 1 dynafocal mounts. O-320-E1B at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as E1A but with retard breaker magnetos. O-320-E1C at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as E1B. O-320-E1F at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as E1C but with propeller governor drive on the left front of the crankcase. O-320-E1J at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as the E1F but with Slick magnetos. O-320-E2A at 2,700 rpm, or at 2450 rpm Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as E1A but with fixed-pitch propeller, 3/8 inch attaching bolts and an alternate power rating of . O-320-E2B at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as E2A but with retard breaker magnetos. O-320-E2C at 2,700 rpm, or at 2450 rpm Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as E2A but -1200 series mags and an alternate power rating of . O-320-E2D at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Similar to E2A but with Slick magnetos and O-235 front. Used in the Cessna 172 I to M models. O-320-E2F at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as E1F but with fixed pitch prop provisions. O-320-E2G at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as E2D but with O-320-A sump and intake pipes. O-320-E2H at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as E2D but with S4LN-20 and -21 magnetos. O-320-E3D at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as E2D but with 3/8 inch propeller flange bolts. O-320-E3H at 2,700 rpm, Minimum fuel grade 80/87 avgas, compression ratio 7.00:1. Same as E3D but with S4LN-20 and -21 magnetos.
O-320-H1AD at 2,700 rpm, Minimum fuel grade 100LL avgas, compression ratio 9.00:1. Integral accessory section crankcase, front-mounted fuel pump external mounted oil pump and D4RN-2O21 impulse coupling dual magneto. O-320-H1BD at 2,700 rpm, Minimum fuel grade 100LL avgas, compression ratio 9.00:1. Same as H1AD but with a D4RN-2200 retard breaker dual magneto. O-320-H2AD at 2,700 rpm, Minimum fuel grade 100LL avgas, compression ratio 9.00:1. Same as H1AD but with provisions for a fixed-pitch propeller.