What are the size specifications for spark plug sockets?

[CarlosFitz]

Spark plug sockets come in two size specifications: 16mm and 21mm. The spark plug mainly consists of a terminal nut, insulator, terminal stud, center electrode, side electrode, and shell. As a crucial component of a gasoline engine's ignition system, it introduces high voltage into the combustion chamber, causing it to jump across the electrode gap to generate a spark, thereby igniting the combustible air-fuel mixture in the cylinder. Under high voltage, the air between the spark plug's center electrode and side electrode rapidly ionizes, forming positively charged ions and negatively charged free electrons. When the voltage between the electrodes reaches a certain level, the number of ions and electrons in the gas increases like an avalanche, causing the air to lose its insulating properties. The gap then forms a discharge channel, resulting in a "breakdown" phenomenon. At this point, the gas becomes a luminous body, known as a "spark." Along with thermal expansion, a "crackling" sound is also produced. The temperature of this electric spark can reach as high as 2000-3000°C, sufficient to ignite the air-fuel mixture in the cylinder's combustion chamber.