Spark Plug Construction

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Let's start out with the basic construction of a spark plug.

A spark plug construction picture is attached below. (how do you display a picture in the body of text? the add img link doesnt seem to work)

Starting at the top, the TERMINAL can come 3 ways:
stud - some wires are made to fit over plugs that don't have a terminal nut on top, the plug is produced with the terminal nut left off.
solid - the terminal nut is permanent and can not be removed. Used particularly in the motorsport and marine industry when there is a lot of movement and vibration and a removable terminal nut could come loose.
removable - the plug comes with a terminal nut, but it can be removed.


HEX - This is the area your socket grabs when removing or installing plug. For automotive applications, plugs usually come with a 5/8 or 13/16 hex. Vehicles prior to about 1980 allow for a 13/16 hex, most after 1980 only allow 5/8.


SEAT - Plugs are available in a tapered seat or with a gasket. The two are not interchangable - in order to use a plug with a tapered seat, your cylinder heads must have been made specifically for the use of a plug with a tapered seat.


REACH - The plug reach is measured from the seat to the end of the threaded are (do not include ground strap in measurement).


THREAD DIAMETER - Accurate Measurement of the cylinder head or removed plug is necessary to determine the plug diameter, which may range form 8mm to 18mm.


GROUND ELECTRODE - Ground electrodes come in a variety of shapes and sizes. They are also called by a variety of names depending on manufacturer, IE: trapezoid cut ground, tapered cut ground, fine wire ground, angled ground, trimmed side electrode, wedge shaped ground, inverted V-tip ground, cut back ground, etc. All have the same purpose, to reduce quenching and shadowing. Ground straps will be discussed in more detail in future technical threads.


CENTER ELECTRODE - A traditional center electrode is 2.5mm. Manufacturers have improved spark plug performance by creating fine-wire, taper cut, necked down and v-power center electrodes. Each of these will be discussed in further detail in future technical threads.


GAP - A spark plugs’ tip temperature and the voltage necessary to fire the plug are directly affected by the gap setting. Most manufacturers set the gap from the factory for that plugs most popular application. Unfortunately, that plug may have hundreds of applications from automobiles to golf carts. Setting the gap for your particular engine is important as insufficient spark plug gap can cause pre-ignition, detonation and even engine damage. Whereas too much gap can result in a higher rate of misfires, loss of power, plug fouling and poor fuel economy. Even if the preset gap is supposed to match your motor, it is always best to physically check that the gap is adjusted properly for your motor prior to installation as the gap may have been changed during shipping.

Further information on proper gapping will be discussed in future technical threads.