Spark plugs

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Spark plug is an element of the ignition system of the combustion engine with spark ignition and it is responsible for creating an electrical spark which ignites air-fuel blend.

Parameters and structure of the spark plug

  • Thermic value – determines plug’s capacity to remove the heat. A spark which removes heat more efficiently is called ”cold” plug and the one which removes heat to a small extent (heats up), is commonly called ”hot” plug. This value is different for the various producers. NGK gives hot plugs low index, and high indicator to cold plugs. Bosch, by contrast, does it conversely.
  • Diameter of thread – commonly measures 18, 14, 12 or 10 mm
  • Length of thread – standard length is 19 or 26,5 mm
  • Wrench size – wrench size recquired to screw in/unscrew the plug. The most common sizes are 20,7, 16 and 14 mm.
  • Number of lateral electrodes – their minimum amount is 1 and maximum 4. Number of electrodes doesn’t define plug’s quality.
  • Material of the main electrode – most frequently it is made of good conductors such as copper, nickel, platinum, iridium
  • Space between electrodes – its preferable value is from 0,3 up to 1.3 mm. If the gap is too big, the spark might not skip forward which will result in blend not ignited. However, too small gap might cause the weld of two electrodes for example as the result of a metal filing penetrating through them.
  • Moment of screwing in – moment of force necessary to screw a plug in. Mostly it measures from 10 to 30 nm. If we exaggerate we might have a difficulty when unscrewing it, in the worst case we might simply wring it.

Plug resistence

Spark plug generates spark about from 200 to 1500 times per minute. Most producers guarantee impeccable functioning of the spark for about 20 millions of cycles, which gives about 15-30 thousands kilometres. Obviously many factors contribute to this number, such as quality and material of plug. For instance a plug with platinum electrode is more durable than the one with electrode made of nickel-lithium alloy.

Plug classification by electrode construction

There are 3 kinds of the plug:

  • Standard plug – plug with electrodes made of nickel-lithium alloy. It is currently the cheapest solution, but also the temporary one.
  • Iridium plug – the end of the middle electrode is made from iridium alloy, which significantly increases plug resistence. Iridium melts just in the temperature of 2,446 degrees Celsius . Additionally, plug size might be smaller considering that use of this material allows to decrease diameter of middle electrode down to 0.4 mm.
  • Platinum plug – durability of the plugs with platinum middle electrode increases period of use up to 4 times comparing with the standard version. Also, voltage generated from solenoid might be lower for this kind of spark plug.

Furthermore increasing the number of mass electrodes boosts durability of plugs because spark might skip forward in turns to any of them.

Petrol vs. Diesel

High-pressure engine (of self-starting ignition) doesn’t recquire spark to ignite blend, because as the name indicates it ignites as the result of air pressuring. In Diesel engines instead of ignition plugs we will find heat plugs which aid starting in cold engine. Their function is to heat up the air in the burning chamber.


It is good for the start to check the condition of our plugs. If we notice yellow discoloration on the ceramic insulator right next to plug corpus it might mean that our plug is slowly reaching its end.

Moreover, appearance of our plugs might actually tell us a lot about the quality of burning in the engine chamber.

How to check if the plug is working?

We unscrew the plug and put it back to ignition wire remembering about closing the circle, i. e. we wind a piece of the wire (quite large one) on thread or corpus of the plug, whilst the other end of the wire must be merged with the mass.

What consequences has driving with malfunctioning plugs?

Obviously aside from uneven functioning of the engine, including immobilizing it in some cases, the main consequence is filtering of unburned fuel to catalytic converter which results in its faster usage.


Plug replacement is demonstrated in the movie below:

We should keep in mind that plugs need to be screwed in carefully. Producers always indicate what force moment is necessary to screwing a plug in. If we do not have torque wrench, we must do this instinctively!

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