What Do I Need To Know About the Lycoming O-320 Light Aircraft Engine?

The Lycoming O-320 is a popular engine with light aircraft. It is a large family of air-cooled four-cylinder direct-drive engines that are rated for either 150 or 160 horsepower. Variants of the engine are used in the Cessna 172, Piper Cherokee, and other planes.

The Piper PA-18-150 Super Cub.
The Piper PA-18-150 Super Cub is typically fitted with a 150-horsepower (112 kW) Lycoming engine, although it is not uncommon to see them equipped with a 160-horsepower O-320-B2B engine variant.

The 320-cubic-inch displacement of the O-320 engine makes it the perfect fit for your smaller airplanes. The engine family also includes the O-320 carburetor, IO-320 fuel injection, reversing bracket, AIO-320 fuel injection, and Aerobatic series AEIO-320 fuel injection.

The LIO-320 is a “left” version with the crankshaft rotating in the opposite direction for use in dual-mount aircraft to eliminate the critical engine.

Similarities with Other Lycoming Engines

The O-320 family of engines is externally similar to the Lycoming family of O-235 and O-290 from which they are derived.

The O-320 shares the same 3.875-inch (98mm) stroke as the smaller engines, but produces more power with the bore increased to 5.125-inch (130mm). The design uses hydraulic tapes and also incorporates provisions for the installation of hydraulically controlled propellers.

Controllable pitch propeller models use a different crankshaft than those designed for fixed-pitch drives.

Inside the O-320

The O-320 uses a standard wet sump system for lubrication. The main bearings, connecting rods, camshaft bearings, taps, and thrust bearings are lubricated, and the piston pins, cylinder walls, and gears are spray lubricated.

The oil system is pressurized by an accessory-mounted oil pump. A flexible hose is used with a remote-mounted oil compressor, connected to the engine.

The 150 hp (112 kW) versions of the carbureted O-320 are homologated for the use of 87 AKI automotive gasoline.

Models with a compression ratio of 9.0: 1, such as the H2AD model, are not allowed. All 160 hp (119 kW) 0-320 are allowed for 91 AKI. Airframe approval is also required for the use of automotive gasoline on any certified aircraft.

“O” or “IO”

All of these engines are essentially the same with one exception: the Model H. So, to begin with, the first letter, “O,” indicates a charred engine. If the letter “I” follows this first number, it indicates an injectable fuel system.

Engine Displacement

The ‘320’ designation indicates a basic “cubic inch” displacement of the engine.

Carburetor and Injection Settings

Lycoming engines may come in “150” or “160” hp models, and then in -A, -B, -C, or -D variants. These letters refer to various combinations of carburetor and injection settings and Type 1 or Type 2 Dynafocal or tapered mount configurations.

The heads (or pants) appear to be machined to produce low compression (150 hp) or high compression (160 hp) versions. Now there are many other variables. For example, all the crankcases are cast on the same base model, but the lugs are machined differently for Type 1 or Type 2 Dynafocal devices.

Almost all carburetors and injectors are located on the bottom and almost all injections are located on the bottom. The exceptions are the O-320-D carb made for the Grumman American Cougar with the carb mounted on top.

Additionally, injectors are mounted on top of Comanche Twin engines (IO-320-A, -B, and -C).

Crankshafts in Lycoming Engines

Lycoming engines may come in O-320-A1, O-320-B1, and O-320-C1 or O-320-A2, O-301-B2, and O-320-C2 variants.

The numbers after the first letter determine whether the crankshaft is hollow (No. 1 for constant speed drives) or solid (No. 2 for fixed-pitch drives).

For variants designated IO-320-A3, IO-320-B3, and IO-320-C3: the front engines (in the IO-320-A1, IO-320-B1, and IO-320-C1 models) were made for a constant speed propeller with 3/8-inch connecting bolts. These had problems so the crankshaft flanges were changed to 7/16 inch. These have been named the IO-320-A3, IO-320-B3, and IO-320-C3, leaving the originals obsolete and no longer used.

Finally, the letter that follows the center number determines the accessories installed. The most common variable is the magnet: Bendix or Slick. Sometimes the letter D is added as in O-320-H2AD. This represents a Bendix dual magnet in a single impeller-driven case.

The exception to all of the above is the “H” series engines, which have struggled. Cessna installed them on some of its Skyhawks, creating a true nightmare for the company. They were soon released by Cessna, many of which made their way into the amateur construction category with mixed success.

(Source: Lycoming)