Click here to upgrade to a newer version of Internet Explorer or Microsoft Edge.
Stay InspiredEAA is your guide to getting the most out of the world of flight and giving your passion room to grow.
Lycoming Nomenclature Made Easy!
July 2015 - Ever wonder where all the letters and numbers for Lycoming engines came from and what they all mean? I recently came across a June 1972 copy of EAAC Canadian Sport Aviation News, where an explanation of the numbers relating to an O-320 Lycoming engine appeared in an article edited from a Falco newsletter by a Mr. Al Constant.
The venerable O-320
It’s all rather confusing, but stay with me.
‘O’ or ‘IO’
All of these engines are basically the same with one exception - the H model. So to start, the first letter, “O,” indicates a carbureted engine. If the letter “I” proceeds this first number, this indicates an injected-fuel system.
This represents the basic “cubic-inch” displacement of the engine, but read on.
‘150’ or ‘160’ hp, A, -B, -C, or -D
Here it gets murky. It appears that the heads (or pistons) are machined to produce low-compression (150 hp) or high-compression (160 hp) versions.
Now there are many other variables. For instance, the crankcases are all cast in the same basic mold, but the lugs are machined differently for Type 1 or Type 2 Dynafocal mounts. Almost all carburetors and injectors are mounted in the bottom, and almost all injectors are mounted in the bottom. The exceptions being the carbureted O-320-D made for the Grumman American Cougar with the carburetor mounted on top. Also the Twin Comanche engines (IO-320-A, -B, and -C) also have the injectors mounted on top.
So these letters refer to various combinations of carburetor and injector locations and Type 1 or Type 2 Dynafocal- or conical-mount configurations.
O-320-A1, O-320-B1, and O-320-C1 or O-320-A2, O-301-B2, and O-320-C2
The numbers after the first letter determine whether the crankshaft is hollow (No. 1 for constant speed propellers) or solid (No. 2 for fixed-pitch propellers).
IO-320-A3, IO-320-B3, and IO-320-C3
The initial engines - IO-320-A1, IO-320-B1, and IO-320-C1 - were made for constant-speed propellers with 3/8-inch attachment bolts. These proved to be problematic, so the crank flanges were changed to 7/16-inch. They were named IO-320-A3, IO-320-B3, and IO-320-C3, making the original ones obsolete and are no longer being manufactured.
Finally, the letter following the central number determines the accessories installed. The usual variable is the magnetos: Bendix or Slick.
Sometimes the letter D is added as in O-320-H2AD. This indicates a Bendix dual magneto in a single-case driven by a single-drive.
The exception to all of the above is the “H” series engines, which proved to be problematic. Cessna installed them in some of its Skyhawks, creating a real nightmare for the company. Cessna soon dropped them, and many found their way into the amateur-built category with mixed success.
Also, did you know that Lycoming made the engine for the Dusenberg J series?
Jack Dueck, Chairman-EAA Canadian Council, EAA 337912