Assignment

The spark plug's job is to reliably ignite the air-fuel mixture in the Otto engine. Thereby, it should reach the self-cleaning temperature of 400°C quickly and at full stress, should not exceed 850°C.

Function

The ignition spark jumps across from a center electrode to one or more earth electrodes. To see this, please click on 'Cut-open earth electrode'! The center electrode is surrounded by an insulator sleeve whose length influences the heat value. If the insulator is short and has only a small breathing area, the heat can be quickly transferred to the thread, and thus to the cooled cylinder head. One speaks of a "cold" plug for engines on which high demands are made and have a high heat value (low heat-range code). Should the insulator sleeve be long, then it's the other way round. The spark plug must be suited to the respective engine. E.g., a spark plug with the description 'W8DC' is a Bosch product for relatively low demand Otto engines.

Material

Insulator from aluminium oxide, electrodes from nickle-, manganese-, chrome- or silver alloy, with platinum on the electron points.

Oldtimer

If you're looking for spark plugs for older vehicles and don't know their description, you could possibly also use a newer type. Take into consideration however, that some manufacturers have changed their descriptions. Bosch, e.g., has even turned the heat range identification number around:

Changing the spark plugs

Great care is to be taken, also in engines whose spark plugs can be reached quite easily. Even loosening them can pose problems, the breaking off of parts of the spark plug can mean a possible re-drilling and/or the tapping of a new thread. Because nowadays actually only aluminium cylinder heads exist, also when screwing them in a lot depends on working precisely, quick fault detection and avoiding the use of any sort of force. Neither oil nor grease should be used as an aid, because the plug could become so tightly lodged that the above problems could occur.

Thread repair

What can be done if the worst comes to the worst?, if the plug tears off and only the screw-thread is left inside? Sometimes one is in luck, the whole inset comes out, also the porcelain section. Then, it's exactly like manufacturing it in reverse. All together, not yet a reason to take the head off, unless the hole to be tapped is difficult to reach. Of course all repair work to the cylinder head is easier when the head has been dimantled. Apart from that, a targeted heating up of the aluminium could make it possible to remove the steel threading from the combustion chamber side. There is also the possibility of using a left-threaded 'easy-out' or a similar, hopefully mild tilting device, to get the thread out. Before drilling, one should ensure that, particularly before inserting proper threaded bushings, that enough material remains so that the cyl. head does not take it's revenge by cracking.
Indeed, a lot of metal shavings are produced and they have the habit of gathering in the combustion chamber. Applying grease onto the tapping device and the liberal use of compressed air, can reduce the shavings to a minimum. Surprisingly enough, there are repairers who are not particularly worried about these shavings in the engine. The fact is however, that even when using a modern endoscope (as a computer extra), not all the shavings can be localised, because e.g., they hide themselves on the side, next to the fire land. So, perhaps it would be better to remove the head? Using the threaded insert is also not without problems. It must be shorter than the original thread, but not by much. This is not only because of the stress, but also, because the sealing surface must not be violated. Now, how does one screw in the wire spiral? Either a suitable guide is provided, and/or, there is a small tongue, (mostly at the end) which, using a suitable tool, is snipped off, without it falling into the combustion chamber. 11/10