Spark plug components

Summary
The main spark plug components are a threaded metal case with a ceramic insulating covering, a sparking tip with an adjustable spark gap, center and side electrodes and a terminal to connect the plug to the ignition system.

Metal case

The metal case removes heat from the insulator and passes it on to the cylinder head. It also provides structural strength to cope with the torque force applied when tightening the plug into place. The case also acts as the ground for the sparks passing through the center electrode to the side electrode.

Insulator

The insulator covering the outside of the metal case is usually made from an aluminum oxide ceramic with a high tolerance to heat and electrical voltage. The ribbing design, the exact composition of the insulator material, and length of the insulator, can all partly determine the heat range of the plug.

Seals

The spark plug seals the combustion chamber of the engine when installed, so the seals make sure that it is kept airtight. The seal is usually a hollow metal washer which at the correct torque setting is partially crushed between the flat surface of the head and the plug just above the threads. These seals should be replaced when plugs are removed and replaced.

Terminal

A terminal at the outer end of the spark plug connects it electrically to the ignition system. Some ignition leads clip on the plug. In other types they may be held in place by a threaded nut.

Side, or ground electrode

The nickel steel side electrode runs very hot and is welded to the side of the metal case. Some spark plug designs have multiple side electrodes that do not overlap the center electrode.

Center electrode

The center electrode is the hottest part of the plug, and it is from here that the electrons that cause the spark are emitted. The electrons stream from the edges of the flat electrode, and as these edges erode and become less sharp, the spark weakens. It used to be necessary to maintain spark plugs by sandblasting corroded surfaces and filing the electrodes sharp again, but as low-erosion materials have improved this is not necessary. Spark plugs now usually last a lot longer and are replaced rather than refurbished.

Spark plug gap

The same plug can sometimes be used in different engines with different gap settings, so the gap should be checked with an accurate feeler gauge and if necessary adjusted according to the specifications in the workshop manual when they are first fitted. The gap is critical to engine performance. A narrow gap may give a spark to weak to ignite the air-fuel mixture effectively. A gap too wide may fail to produce any spark at all. Either way, the wrong gap will reduce engine power and fuel efficiency.

Heat range

The operating temperature of a spark plug refers to the temperature at the sparking tip of the plug inside a running engine. If the tip is too hot, pre-ignition can occur. If it is too cold, deposits may form on the insulator causing spark energy loss. 'Hot' rated spark plugs are better insulators, keeping more of the engine heat in the tip of the plug. A 'cold' rated plug conducts more of the heat out of the spark plug tip. The heat range of a spark plug is affected by a wide range of characteristics, including its design profile and the types of material used in its manufacture.

A spark plug is said to be "hot" if it is a better heat insulator, keeping more heat in the tip of the spark plug. A spark plug is said to be "cold" if it can conduct more heat out of the spark plug tip and lower the tip's temperature. Whether a spark plug is "hot" or "cold" is known as the heat range of the spark plug. The heat range of a spark plug is typically specified as a number, with some manufacturers using ascending numbers for hotter plugs and some using descending numbers for hotter plugs.
The heat range of a spark plug (i.e. in scientific terms its thermal conductivity characteristics) is affected by the construction of the spark plug: the types of materials used, the length of insulator and the surface area of the plug exposed within the combustion chamber. For normal use, the selection of a spark plug heat range is a balance between keeping the tip hot enough at idle to prevent fouling and cold enough at maximum power to prevent pre-ignition leading to engine knocking. By examining "hotter" and "cooler" spark plugs of the same manufacturer side by side, the principle involved can be very clearly seen; the cooler plugs have more substantial ceramic insulators filling the gap between the center electrode and the shell, effectively carrying off the heat, while the hotter plugs have less ceramic material, so that the tip is more isolated from the body of the plug and retains heat better.
Heat from the combustion chamber escapes through the exhaust gases, the side walls of the cylinder and the spark plug itself. The heat range of a spark plug has only a minute effect on combustion chamber and overall engine temperature. A cold plug will not materially cool down an engine's running temperature. (Too hot of a plug may, however, indirectly lead to a runaway pre-ignition condition that can increase engine temperature.) Rather, the main effect of a "hot" or "cold" plug is to affect the temperature of the tip of the spark plug.
It was common before the modern era of computerized fuel injection to specify at least a couple of different heat ranges for plugs for an automobile engine; a hotter plug for cars which were mostly driven mildly around the city, and a colder plug for sustained high speed highway use. This practice has, however, largely become obsolete now that cars' fuel/air mixtures and cylinder temperatures are maintained within a narrow range, for purposes of limiting emissions. Racing engines, however, still benefit from picking a proper plug heat range.

Reading spark plugs

The spark plug's firing end will be affected by the internal environment of the combustion chamber. As the spark plug can be removed for inspection, the effects of combustion on the plug can be examined. An examination, or "reading" of the characteristic markings on the firing end of the spark plug can indicate conditions within the running engine. The spark plug tip will bear the marks as evidence of what is happening inside the engine. Usually there is no other way to know what is going on inside an engine running at peak power. Engine and spark plug manufacturers will publish information about the characteristic markings in spark plug reading charts (e.g. a general spark plug reading chart).
A light brownish discoloration of the tip of the block indicates proper operation; other conditions may indicate malfunction. For example, a sandblasted look to the tip of the spark plug means persistent, light detonation is occurring, often unheard. The damage that is occurring to the tip of the spark plug is also occurring on the inside of the cylinder. Heavy detonation can cause outright breakage of the spark plug insulator and internal engine parts before appearing as sandblasted erosion but is easily heard. As another example, if the plug is too cold, there will be deposits on the nose of the plug. Conversely if the plug is too hot, the porcelain will be porous looking, almost like sugar. The material which seals the center electrode to the insulator will boil out. Sometimes the end of the plug will appear glazed, as the deposits have melted.
An idling engine will have have a different impact on the spark plugs than one running at full throttle. Spark plug readings are only valid for the most recent engine operating conditions and running the engine under different conditions may erase or obscure characteristic marks previously left on the spark plugs. Thus, the most valuable information is gathered by running the engine at high speed and full load, immediately cutting the ignition off and stopping without idling or low speed operation, and removing the plugs for reading.
Spark plug reading viewers, which are simply combined flashlight/magnifiers, are available to improve the reading of the spark plugs.

Two spark plug viewers

Once again, however, the practice of reading spark plugs has largely become obsolete now that cars' fuel/air mixtures and cylinder temperatures are maintained within a narrow range, but is still valuable for racing applications.

Indexing spark plugs

A matter of some debate is the "indexing" of plugs upon installation, usually only for high performance or racing applications; this involves installing them so that the open area of the spark gap, not shrouded by the ground electrode, faces the center of the combustion chamber, towards the intake valve, rather than the wall. Many experts believe that this will maximize the exposure of the fuel-air mixture to the spark, and therefore result in better ignition; others, however, believe that this is useful only to keep the ground electrode be out of the way of the piston in ultra-high-compression engines if clearance is insufficient. In any event, this is accomplished by marking the location of the gap on the outside of the plug, installing it, and noting the direction in which the mark faces; then the plug is removed and additional washers are added so as to change the orientation of the tightened plug. This must be done individually for each plug, as the orientation of the gap with respect to the threads of the shell is random.
Spark plugs should operate between average temperatures of 40°c and 80°c or 104°F and 176°F. This is referred to as the heat range. The temperature that a spark plug will reach depends on the distance that the heat must travel from the insulator to reach the outer shell of the plug and enter the cylinder head and the water jacket. If the heat path is long, the plug will retain more heat and therefore will run at a higher temperature than one with a short heat path.