This factor is necessary for the correct selection of the installation tool.

There are four types:

1. M10 × 1 - for motorcycles, chainsaws, lawn mowers.
2. M12 × 1,25 - for motorcycles.
3. M14 × 1,25 - for most modern cars.
4. M18 × 1,5 – for cars of early manufacture years, cars with gas fueled reciprocating engines.

As a rule, there are three groups of spark plugs based on the thread length:

1. Short thread - 12 mm.
2. Medium thread - 19-20 mm.
3. Long thread - 25 mm.

The more powerful the engine, the longer must be the thread of the spark plug. A long thread creates a large contact area of the spark plug with the engine cylinder head, and therefore a large heat exchange coefficient.

The distance between the electrodes of spark plugs ranges from 0.5 mm to 2 mm. The greater the distance between the electrodes of the spark plug the better is the combustion of fuel mixture. However, it is worth remembering that for spark plugs with a bigger gap between electrodes it takes a large punch through voltage, which increases the probability of a misfire, the amount of harmful substances in exhaust gases and fuel consumption. Therefore, the optimum gap between the electrodes is determined by the manufacturer. It is not recommended to change it independently.