FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsEasy
A technician installs a spark plug whose reach (thread length) is longer than that specified for a particular cylinder. What is the most likely result?
- AThe plug will not fire because secondary voltage cannot reach the electrode
- BThe electrode gap will widen at an accelerated rate
- CThe exposed threads inside the combustion chamber can accumulate carbon and may be struck by the piston or valves
- DThe plug's heat range will automatically decrease, causing fouling
Show answer & explanationAnswer & explanation
Correct answer: C. The exposed threads inside the combustion chamber can accumulate carbon and may be struck by the piston or valves
Spark plug reach must match the cylinder head boss depth. A plug with excessive reach protrudes too far into the combustion chamber, exposing unshielded threads to combustion gases where carbon deposits form and interference with the piston or valves can occur.
Why the other options are wrong
- A. Reach does not affect the ability of secondary voltage to jump the gap.
- B. Reach does not directly affect gap wear rate; that is governed by electrode erosion from arcing.
- D. Heat range is a design property of the plug, not something changed by reach.
Spark Plug Reach
The length of the threaded shell that engages the cylinder head, which must match the manufacturer's specification so the electrode sits at the correct depth in the combustion chamber.
- Too short a reach recesses the electrode, causing poor ignition and fouling in the recess
- Too long a reach exposes threads to combustion gases, risking carbon buildup and piston/valve contact
- Reach specifications are given by engine and cylinder manufacturer, not by plug heat range
Memory trick: Reach too long, threads go wrong — carbon and clash await inside the wall.