Spark Plug Technical Terms Glossary

This is the diameter of the plug threads measured across the thread as pictured. Standard pitches for various thread diameters are listed below but it is by no means guaranteed that a standard diameter thread will use a standard pitch size.

The pitch is the measurement between a pair of thread peaks or troughs. Standard thread and pitch is listed below but there may be exceptions.

M8 x 1.0mm
M10 x 1.0mm
M12 x 1.25mm
M14 x 1.25mm
M18 x 1.5mm

The plug's seat is the part which 'mates' with the cylinder head of the engine. There are two simple types of seat.

1. Tapered/conical, where there is no gasket or washer, contact is made directly with the cylinder head. Both plug and head mating surfaces need to be maintained in perfect condition to allow an efficient seal.
2. Flat/gasket, where a gasket (also known as crush washer) is used to aid sealing between plug and cylinder head.

The tip configuration of the plug refers to its 'projection' - that is the amount that the central electrode and, to some extent, the ceramic insulator of the central electrode, protrudes from the threaded shell of the spark plug. A projected plug will normally offer more efficient combustion, but beware, too much projection can promote plug damage and subsequent engine damage as well as the possibility of making physical contact with piston crowns and valves (which is also likely to cause plug/engine damage).

We have categorised plugs as being:

1. Recessed - the firing tip is entirely within the threaded shell, offering extreme protection for racing engines, good vibration resistance but prone to fouling.
2. Non projected - the tip of the centre electrode is more or less level with the end of the threaded shell. The ground electrode protrudes from the end of the shell but is relatively short and therefore provides good vibration resistance and good thermal conductivity from the electrode tip. We normally also categorise surface discharge types as non projected.
3. Projected - centre electrode and it's insulator protrude from the threaded shell to some extent (usually around 1.5mm). This means the tip is more exposed to the combustion process and tends to keep cleaner, it also means that the spark is produced in a position closer to the centre of the combustion chamber - where the greatest efficiency can usually be achieved.
4. Extra projected - same as projected but projected further. This could be anything from 2.5mm to 10mm or more. These plugs are normally designed for specific applications and use in other applications is highly likely to result in engine damage. Not recommended unless explicitly listed as suitable by the engine or spark plug manufacturer. A non threaded 'shroud' section is sometimes included at the end of the thread to afford some protection to the otherwise exposed insulator.

The terminal is the part of the plug to which the plug cap is connected. There are a number of types of terminal as follows (most common first):

1. Removable (terminal nut). This type comprises a threaded terminal (see below) as well as a fitted terminal nut (also referred to as a 'nipple' or 'ferrule'). Can be used with or without the terminal nut to provide a suitable connection to most plug caps (apart from special applications). The downside with this type is that the terminal nut can work loose and potentially cause electrical continuity issues. A blob of 'superglue' on the terminal threads before fitting the terminal nut can provide a little more security for critical applications. Do not overtighten the nut as the plug may become damaged. Standard size for a terminal nut is an M4 thread, Industrial spark plugs and igniters may use a different thread.
2. Solid/Fixed/Post terminal. This type has the appearance of a fixed 'terminal nut' (see above) which cannot be removed (do not try, you will damage your plugs!)
3. Threaded terminal. Exposed terminal threads, no terminal nut is included by the manufacturer. If you have purchased plugs with a threaded terminal but require a solid terminal or terminal nut to make connection with your plug caps possible then you are most likely using the incorrect plug cap type or an incorrect plug. We can normally supply terminal nuts to our customers on request, please specify with your order if you need terminal nuts.
4. Special/Stud etc. Any terminal which does not fall under the categories above is 'special' - i.e. it will need a special plug cap. This applies mostly to miniature and sub miniature model engine plugs and certain NGK race plug designs.