Definition
The location on a reciprocating engine cylinder where the cylinder head temperature (CHT) thermocouple, or a temperature-sensing gasket, is installed. It is the standard reference point on the cylinder barrel just beneath the lowest cooling fin, used to obtain a representative reading of cylinder operating temperature.
Plain English
It is the spot on the outside of an engine cylinder, just under the bottom cooling fin, where the temperature sensor is fitted. Putting the sensor here gives a reliable reading of how hot that cylinder is running.
Context Anchor
Seen in piston-engine maintenance instructions, inspection descriptions, and temperature-probe location guidance.
Derivation
Cylinder comes from an old Greek word meaning a roller or round body. Barrel is used because the main cylinder section is round and tube-like. Fin is used because the thin metal ribs stick out like small fins and help move heat into the air.
Why Pilots Care
CHT is one of the most important indicators of engine health. The reading is only meaningful if the sensor is in the correct location, so pilots and mechanics need to know that this specific spot, just below the bottom cooling fin, is the standard reference point.
Grounding Statement
Picture the ribbed outside of an air-cooled engine cylinder; this location is just below the lowest rib on the round cylinder wall.
Intuition Check
Do not read barrel as a gun barrel or storage barrel here; it means the round sleeve of the engine cylinder. Do not read fin as an aircraft tail surface here; it means a cooling rib on the engine.
Example Sentence 1
The mechanic installed the CHT thermocouple on the cylinder barrel, slightly below the bottom cooling fin, as specified in the engine manual.
Example Sentence 2
Corrosion often appears first on the cylinder barrel slightly below the bottom cooling fin where airflow is reduced.