Definition
Thin metal ridges or vanes cast or attached to the outside of an air-cooled engine's cylinders that increase the surface area exposed to the airflow, allowing heat from combustion to transfer more rapidly into the passing air.
Plain English
The thin metal ribs sticking out from each cylinder. They give the air more metal to flow over, so the engine sheds heat faster and stays at a safe temperature.
Context Anchor
Seen in discussions of air-cooled piston engines, engine cooling, preflight inspection, and engine temperature control.
Derivation
From Old English finn, meaning the projecting blade-like part on a fish. Engineers borrowed the word because the cylinder's ridges look and function like fish fins -- thin projections that interact with a surrounding fluid (in this case, air).
Why Pilots Care
Well-designed fins keep cylinder head temperatures within safe limits and prevent overheating that can cause detonation or engine damage.
Grounding Statement
As air flows over the fins, heat moves from the hot metal into the passing air and away from the engine.
Intuition Check
Do not think of fins here as steering surfaces or fish parts. In this engine-cooling context, fins are metal cooling surfaces that help remove heat.
Example Sentence 1
During preflight, the pilot checked the cylinder fins for cracks and removed a small piece of grass lodged between them.
Example Sentence 2
Oil and dirt buildup on the cooling fins reduced airflow and caused higher cylinder temperatures in flight.