Definition
The length of the threaded portion of a spark plug shell that screws into the cylinder head. Reach must match the cylinder's spark plug bore depth so the firing end of the plug sits at the correct position in the combustion chamber.
Plain English
How far the threaded part of a spark plug extends into the engine cylinder. The plug's threads have to be the right length for the hole they screw into.
Context Anchor
Seen when selecting, inspecting, or replacing aircraft engine spark plugs using the engine maintenance manual or approved parts data.
Derivation
From the everyday sense of 'reach' meaning 'how far something extends.' Here it refers to how far the plug's threads reach into the cylinder.
Why Pilots Care
The wrong reach can place the electrode too far in or too far out, causing detonation, overheating, or loss of power.
Analogy
Think of choosing a bolt for a threaded hole. If the bolt is too short, it does not use the hole correctly; if it is too long, it can stick out where it should not. Spark plug reach is the same kind of length match.
Intuition Check
Reach does not mean how far the spark travels. Here, reach means the physical length of the threaded part of the spark plug that goes into the engine.
Example Sentence 1
The mechanic checked the engine specifications to confirm the correct reach before installing the new spark plugs.
Example Sentence 2
Installing a longer-reach plug than required can cause the electrode to contact the piston.