The SPARK PLUG was invented by African-American inventor Edmond Berger on February 2, 1839. It is used to ignite the fuel/air mixture in most petrol engines, and so as a device is essential to the world economy as we know it. They are used in two and four-stroke engines, as well as wankel rotary engines.

There is very little nonstandard terminology to consider when regarding a spark plug. The only thing one needs to know is that the portion of the isolator surrounding the engine side of the core is known as the "core nose". It is also known as the "insulator tip" or "insulator nose".


                           +-- 6-sided wrench flats for socket
           Ceramic         |
           Isolator        |     Threads
                 |         v        |
Terminal         v        ________  v
 |                       |    |   ////=======
 |     ________XXXXXXXXXX|____|   ////XXX_  W       Outer
 +--> (___|____XXXXXXXXXX|    |   ////XXX_| W <-- Electrode
               XXXXXXXXXX|____|   ////XXX
            ^            |____|___////    ^
            |                          ^  |     Center
            +-- Core                   |  +--- Electrode
                                       +-- Core Nose
                                        (Insulator Nose) 

(Please pardon my ASCII art, I am only trying to get the point across.)

All spark plugs share a common basic design; They are generally about three to five inches in length overall. They feature a metallic core, surrounded by an insulator, usually made from ceramic for its ability to withstand heat and pressure. Wrapped around this insulator (or isolator) is an externally threaded metal piece from which an electrode approaches the core, on the end of the plug which resides within the engine. The other end of the core consists of a terminal which is connected either to a wire, on most vehicles, or to a plug on an ignition coil in coil-on-plug (COP) ignition systems. The terminal is generally threaded, with a small adapter screwed on in most cases to accept a slip-on connector. This adapter is generally removed in the case of small engines and/or in high vibration situations, such as many lawn mowers, chainsaws, and the like, as well as some dirt bikes.


Spark plugs are generally installed using a spark plug socket and a socket wrench. The spark plug socket is a normal deep socket with a small rubber piece inside of it which is intended to grab onto the terminal and hold the plug inside of the socket while you manouver it into position. Spark plugs are often in somewhat inaccessible locations, and so you will frequently need at least a wobble bar extension, if not an articulated one. The plug is simply inserted into the spark plug socket, and screwed into the head (typically) of the engine.

The size of the gap between the core and electrode of a spark plug is significant, and the manufacturer of a car will specify a gap, typically in hundredths or thousandths of an inch. One uses a gapping tool to set the gap properly. There are two basic types of gapping tool. One resembles a coin and has a taper roundabout its outer edge, with the size marked in graduations around the edge. One slides the thinnest part of the tool into the gap, and rotates it until the proper gap is forced by the lever action of the tool. The other is the leaf type, and it consists of a number of leaves of metal which can be folded out and which have their sizes stamped on them. The proper leaf is inserted between core and electrode, and the electrode is thus forced to the proper distance from the core. Spark plugs come pre-gapped, but they may not be pre-gapped for your application, as one plug is generally used in many engines.

According to NGK, you should reduce the gap about four thousandths of an inch for every 50 horsepower you add due to increasing compression, either via ram air, nitrous oxide injection, or a turbocharger or supercharger. However, if you increase the power of your ignition system, perhaps by simply upgrading your coil, or by installing an entirely new high-power ignition system like those sold by MSD, Crane, Nology, or Accel, you should open the gap by two to five thousandths of an inch.

Spark plug threads are some of the most-frequently stripped. To avoid this, make sure to check the torque specifications before reinstalling them, and use a torque wrench when you perform the reinstallation. Like most anything on a car, you should disconnect the battery (negative terminal first) before performing any work. Spark plugs are installed in the head, the hottest part of the engine since it includes the exhaust valves, and they can be extremely hot. If you drop them, you will likely damage something; either you will bend the electrode, which must be re-gapped; crack the ceramic, in which case the plug must be replaced to avoid creating a spark on the outside of the plug; or you will damage the threads of the plug, which can be repaired with a triangular file.


Spark plugs are, unsurprisingly enough, used to generate a spark. This spark's purpose is to ignite the fuel/air mixture inside the combustion cylinder of a motor, which pushes down the piston (and thus the connecting rod or conn rod), in order to turn the crankshaft. Naturally, this is a dangerous environment, and wear and tear limits the lifespan of a spark plug to about two years in most practical applications. Damaged or dirty ("fouled") plugs will inhibit generation of a proper spark, and will cause your car to run inefficiently, or not at all. Voltage for the spark is provided by the ignition coil, and is somewhere in the tune of twenty to fifty thousand volts, depending on the system.


The "heat range" of a spark plug refers to the heat-handling ability of the spark plug. Spark plugs must be run between 500°C and 850°C. A hot plug has more exposed insulator surface, so it dissapates heat slowly, thus heating up more quickly. A cold plug has less insulator surface exposed, and so it heats up more slowly, dissipating more heat over time.

If you raise the compression ratio of your engine, through performance-oriented modifications, you will need to run a colder plug, since raising compression raises the cylinder temperature, and you will otherwise raise the heat of the plug, potentially damaging the plug, and causing detonation. NGK suggests that for every 75-100 horsepower you add, you should go one step colder.

If you are experiencing severe oil or fuel fouling, you may need to move to a hotter plug, as your spark may not be occurring correctly. However, it is probably better to inspect plug wires and ignition timing.


The appearance and condition of your spark plugs can tell you a considerable amount about conditions inside of your engine.

A normally worn plug will have an electrode, core, and core nose which are coated somewhat evenly with gray or brown deposits. The gap will have increased by approximately one thousandth of an inch for every thousand miles driven. A plug in this condition indicates that your car is tuned correctly, and that you are using the correct plugs.

A plug covered in sooty black deposits has been fouled with carbon. This inhibits the spark, and will cause misfires, due to a weak spark. This condition is caused by an engine which is running too rich.

A plug which has been coated with an oily black residue is indicative of worn bores or piston rings, allowing an excess of oil into the combustion chamber. All cars burn oil, but this definitely shows that you are vaporizing too much of it. This will also cause misfires and a weak spark.

A spark plug which appears glazed, and has a white core nose with very few deposits is indicative of overheating, and a pre-detonation condition. Your plug may be in the wrong heat range, your ignition timing might be advanced too far, you may be running fuel with too low an octane rating, or you may be running lean.

Plugs with a damaged electrode or core, and a burned or cracked core nose are indicative of overheating and detonation. This can be caused by any of the same conditions which will cause your plug to glaze, or by your cooling system being inadequate or malfunctioning. This condition indicates that you are causing severe damage to your engine, and you should immediately attempt to determine the cause, or you may destroy your engine.

In any case other than plugs which are wearing normally, you should discard the plugs rather than attempting to clean them.


The answer is: Usually not.

In almost every case, the best spark plug for your car is the stock plug. Your engine and ignition system have certain characteristics, and a plug is chosen to make the best of them.

Many "special" spark plugs are on the market today, featuring various coatings (platinum is most common) and designs, especially a V-shaped electrode (on "splitfire" plugs) and multiple electrodes (most notably the Bosch Platinum+4 plugs). But consider the basic facts of an ignition system; except in special situations, such as an aftermarket ignition system from MSD or similar, there is only one spark occurring per cylinder per cycle. How will multiple electrode help this? You will only get a spark jumping from one of them to the core electrode.

In many cases, switching to a plug such as those will actually hurt performance, especially in modern cars with highly tuned ignition systems. By the same token, if your car specs a platinum plug stock, you should definitely use that plug. In almost every case, the only thing you might need to do is pick a plug with a different heat range (described above.)


  1. Webpage: Automobile History - Car Innovation History. (
  2. Webpage: Read Your Plugs. Don Nimi, PDM Racing. (
  3. Webpage: Spark Plugs Overview. NGK. (

I thought I'd share a few things that I've learned about checking and changing spark plugs. This seems like a simple task, but after a few recent projects, I've found that it isn't so straight forward.

Don't take them out of the engine when it is hot. You WILL burn yourself. You may be thinking that you will be careful and hold them in the socket or wear gloves, but you will end up touching them, and they will burn you.

Once this happens, you will drop them on the floor. You may think that you're tough and the burn won't faze you too much, or that you would just set them down quickly, but you won't; you will drop them.

And when you drop a spark plug, there is only one way for it to land. That, of course, is on the electrode, and it will bend. It may land on a towel, or in the grass, or maybe even in a pile of cotton, but the electrode will hit first and it will bend. Don't ask me why, but it ALWAYS happens this way.

After you bend the electrode and you finally dig out your gap checker to fix it, you will start to fumble with it, and drop it again, and it will bend again, and you will have to reset the gap... again.

So, just to make your life a little easier, wait a few minutes and let the plugs cool so that you don't have to laugh at yourself for bending them... twice. Hope this was useful to someone :)

Spark plug.

In internal-combustion engines with electric ignition, a plug, screwed into the cylinder head, having through it an insulated wire which is connected with the induction coil or magneto circuit on the outside, and forms, with another terminal on the base of the plug, a spark gap inside the cylinder.


© Webster 1913.

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