The spark plug consists of a central electrode, an insulator with a housing and a ground electrode. The mass electrode is hermetically fixed in the insulator, and the insulator is rigidly connected to the housing. A spark jumps between the central electrode and the ground electrode, which ignites the air-fuel mixture. Ease of starting the engine, its behavior in idling mode, acceleration and maximum speed of the car depend on spark plugs. Therefore, one should not, without reason, deviate from the type of candles recommended by the manufacturer, which is characterized by a glow number. Heat rating is the degree of resistance of the spark plug to thermal stress in the engine under various operating conditions. Spark plugs are selected for the engine so that their self-cleaning temperature is reached under all conditions. The lower the glow number of the spark plug, the higher the resistance to glow ignition and the less resistant it is to contamination. The higher the glow number of a spark plug, the lower the resistance to glow ignition and the greater its resistance to contamination.