Spark plugs differ in the following things:
The usual threads are M10, M12 and M14 Classical vehicles usually have an M14 thread. 4-stroke and modern 2-stroke with injection use the smaller plugs with M10 or M12.
The common thread lengths are 19mm (long thread)and 12.7mm (short thread).
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 spark plug is for engines which develop a very high combustion chamber temperature, usually these are engines which have a high power output measured by their displacement. A hot spark plug, on the other hand, is used in series production engines with low loads. This type of candle quickly reaches a high temperature and 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, but with a cold candle exactly this is strongly prevented. The heat value has nothing to do with the strength or performance of a spark plug. A cold spark plug in a cold (weak) engine will oil or soot and can therefore break without damaging the engine itself. A hot spark plug in a racing engine burns up and damages the engine ,usually severely.
The heat value of a spark plug is not standardized. Every manufacturer has its own code.
- 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
Here comes copper, platinum, silver or even iridium.
Copper is the most common, 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.