Spark plug -NGK BR ES- online

Spark plugs differ in the following things:

Thread diameter
The usual threads are M10, M12 and M14. Classic vehicles usually have an M14 thread. 4-strokes and modern 2-strokes with injection use the smaller plugs with M10 or M12.

thread length
The common thread lengths are 19mm (long thread)and 12.7mm (short thread).

heat value
The heat value is decisive for the thermal fatigue strength of the spark plug and its self-cleaning effect. A distinction is therefore made between so-called cold and hot spark plugs.
A cold plug is for engines that develop a very high combustion chamber temperature, usually these are engines which, measured by their cubic capacity, have a lot of power. A hot candle, on the other hand, is used in production engines with low loads. This type of candle quickly reaches a high temperature, whereby the soot on the candle burns off well.
The temperature resistance is achieved by differently designed insulators. A cold candle can give off a lot of temperature via the cylinder head, and a cold candle can strongly prevent this. The heat value has nothing to do with the strength or efficiency of a spark plug. A cold candle in a cold (weak) engine becomes oily or sooty and can break without damaging the engine itself. A hot spark plug in a racing engine burns up and damages it, usually severely.
The heat value of a spark plug is not standardized. Each manufacturer has its own code here.

• NGK from cold to hot = 10-9-8-7-6
• Bosch from cold to hot = 2-3-4-5-6
• DENSO from cold to hot = 37-34-31-27-24-22-20-16-14

Electrode material
Here copper, platinum, silver or iridium come to the electrode materialbr />Here copper, platinum, silver or even iridium come to the electrode material
The most common is copper, followed by silver and platinum. Due to the high prices for the precious metals, the spark plugs are also correspondingly more expensive than their copper counterparts.
The material is ultimately almost only interesting for wear, only the extremely thin Iridium electrodes (Ø=0.6mm) have a higher ignition voltage, the spread of the flame front in the combustion chamber improves.

(machine-translated)