Definition
The small, specified distance between the center electrode and the ground electrode of a spark plug, set to a manufacturer-defined measurement so the ignition spark jumps cleanly across the gap and ignites the fuel/air mixture in the engine cylinder.
Plain English
The exact tiny space between the two metal points inside a spark plug. The space has to be set to the size the engine maker specifies so the plug sparks properly.
Context Anchor
Encountered during preventive maintenance when spark plugs are inspected, cleaned, replaced, or adjusted before being returned to service.
Derivation
‘Gap’ comes from Old Norse meaning an opening or empty space. ‘Clearance’ comes from Latin clarus, meaning clear or open. Together they describe the open space the spark must cross — and reinforce that this space is measured, not guessed.
Why Pilots Care
A gap that is too wide or too narrow produces weak or missing sparks, leading to rough running, hard starts, or in-flight engine roughness.
Intuition Check
Spark plug gap clearance does not mean general space around the spark plug. It means the exact small distance between the two metal firing tips of the plug.
Example Sentence 1
During the 100-hour inspection, the mechanic checked each spark plug gap clearance with a feeler gauge and adjusted any that were out of tolerance.
Example Sentence 2
If spark plug gap clearance is set too wide the engine may misfire during a go-around.