Definition
A spark plug with a long heat-transfer path from the firing tip to the cylinder head, causing the tip to retain heat and run at a relatively high temperature during engine operation. Hot plugs are used in low-compression or low-power engines that tend to run cool, where the elevated tip temperature is needed to burn off combustion deposits and prevent fouling.
Plain English
A spark plug designed to hold onto heat at its tip so it stays hot enough to keep itself clean. It's used in engines that naturally run cool and would otherwise foul the plugs with deposits.
Context Anchor
Seen in aircraft engine maintenance, spark plug selection, and troubleshooting rough engine operation caused by dirty or misfiring plugs.
Derivation
Called 'hot' because of how the plug behaves thermally during operation -- the firing tip runs hotter than that of a 'cold' plug. The label refers to the plug's heat range, not to anything about the spark itself.
Why Pilots Care
Selecting the correct heat range prevents plug fouling on the ground or pre-ignition in flight; the wrong choice can cause rough running or engine damage.
Intuition Check
Do not read “hot spark plug” as “a plug with a hotter spark.” It means the plug’s firing tip operates at a higher temperature.
Example Sentence 1
The mechanic installed hot spark plugs in the low-compression trainer engine to reduce fouling during extended ground operations.
Example Sentence 2
Switching to a hotter plug helped stop the fouling the owner had been seeing after every short flight.