FAA Powerplant Written Practice Test

What does the term "reach" refer to in spark plug design?

1. Distance from the shell gasket seat to the end of the threads

2. Distance from the firing end of the plug to the cylinder head

3. Distance the spark plug extends into the combustion chamber

4. Distance between the electrodes of the spark plug

The correct answer is: Distance from the shell gasket seat to the end of the threads

The term "reach" in spark plug design specifically refers to the distance from the shell gasket seat to the end of the threads. This dimension is crucial because it helps ensure that the spark plug is positioned correctly within the combustion chamber for optimal performance. Having the proper reach allows the spark plug to be at the right depth to effectively facilitate combustion, ensuring that the spark will ignite the air-fuel mixture at the right moment. Correct reach is vital for proper engine timing, performance, and avoiding issues such as pre-ignition or fouling. An incorrect reach can lead to inadequate combustion efficiency and could potentially harm the engine. The reach ensures that the spark plug can perform its function effectively without causing interference with the piston or valves. Other dimensions and distances related to the structure of the spark plug, such as the distance from the firing end to the cylinder head or the distance between electrodes, while important for different characteristics like ignition timing and energy discharge, do not define the term "reach."