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Spark Plug Technical Terms
Spark Plug Thread Diameter |
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This is the diameter of the plug's thread measured across the thread as pictured. Pitch is the measurement between a pair of thread peaks or troughs. Standard thread diameters and the related standard pitch sizes are listed below. Note that there may be exceptions. M8 x 1.0mm |
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Spark Plug Thread Reach |
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The thread reach of a spark plug is measured, and usually quoted in mm, from the plug's base (where it mates with the cylinder head) to the maximum extent of the thread (not including the ground electrode or any shroud*). A seat taper or gasket is not normally included in this measurement. For the purposes of measurement, the gasket should be ignored. *At the lowest point of the spark plug thread there may be a non threaded or shrouded portion. The purpose is usually to allow the electrodes to project further into the combustion chamber without exposing too much of the central insulator material to the combustion process, thereby the risk of insulator damage is reduced. |
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Spark Plug Seat Type |
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The spark plug seat is the part which seals against the engine's cylinder head. There are two simple types of seat:
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Spark Plug Hex Or Spanner Size |
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Hex size is the 'spanner size' of the fitting/removal nut, normally measured 'across flats' and normally specified in mm. Hex sizes of some older plugs or US parts are quoted in Inches. Note that some tool manufacturers label spark plug sockets and spanners based on thread size rather than spanner size, this is an outdated an illogical practice as there are several different hex sizes possible for some thread diameter sizes. Always check the spanner size matches the plug hex size - NOT the thread size. |
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Spark Plug Electrode Or Tip Projection |
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The Projection of the plug refers to the amount that the central electrode and, to some extent, the ceramic insulator of the central electrode, protrudes from (or is recessed within) 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 categorise spark plug tip projection as follows: |
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| Recessed - the firing tip is entirely within the threaded shell, offering extreme protection against vibration damage for racing engines. More prone to fouling and cold starting problems than projected spark plugs. Top image is an NGK racing plug with platinum bar ground electrode and gold palladium centre electrode just visible. Bottom image is a Champion retracted gap plug. A 'peg' passes through the side wall of the threaded shell, just visible on the image. The centre electrode is not visible, being well within the threaded shell. Both of these types have now been retired with little or no stock remaining. Iridium racing plugs are the only viable replacement. |
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| Surface Discharge or Surface Gap spark plugs. The ultimate way to prevent electrode vibration damage is to remove the ground electrode entirely. The spark fires from the metal shell, which acts as the ground electrode, to the central electrode, which has little or no projection past the end of the threaded shell. Because the spark flashes across the ceramic central insulator it assits in spark plug 'self cleaning', by burning away deposits. |
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| Non projected The tip of the centre electrode is protrudes onlyone or two mm past 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. |
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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. |
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| 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. |  |
Spark Plug Terminal Type |
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| Removable terminal nut. This type comprises an M4 threaded terminal (see below) as well as a fitted terminal nut (sometimes referred to as a ferrule). Can be used with or without the terminal nut to provide compatibility with most spark plug caps. The downside with this type is that the terminal nut can potentially 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. Industrial spark plugs and igniters may use a different thread |  |
| Solid/Fixed/Post/SAE/Crimped terminal. This type has the appearance of a fixed 'terminal nut' (see above) which cannot be removed (do not try, you will damage your plug!) Note that some spark spark plugs have a 'crimped' terminal, a terminal nut which has been mechanically secured to the M4 thread beneath. For practical purposes this type can be considered a fixed terminal nut, not removable despite appearances. |  |
| 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. |  |
| Racing Special/Stud. The image shows NGK R7282-* (also applies to 7282A-*) where * is the heat range. This plug type has a special stud type connector design, only compatible with NGK racing spark plug cap TRS1225-B and its equivalents. |  |
| Model Engine Special/Stud. This image shows NGK spark plug special stud type connector for sub miniature model engine spark plug CM-6. |  |
| Knurled Nut/Thumb Nut. This type of terminal is designed to accept an HT cable terminated with a brass 'eye' for vintage applications. The knurled nut is removed, the eye installed over the thread and the nut is simply tightened by hand. You may prefer to nip it a little tighter with suitable pliers for a little added security. Note that there are different sizes of thread used for this terminal type, it's critical to use a nut with matching thread. The safest option is to use the nut originally supplied with the spark plug. |
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The terminal is the part of the spark plug where the connection is made to a spark plug cap, boot or direct fire (plug top) ignition coil. There are a number of types of terminal as follows.
