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by Butch Thomas  More from Author

How To Read And Service Spark Plugs

Many of us change the spark plugs in our collector car or everyday ride, but give little thought to them. The spark plug needs to be honored with more importance though – it is the final component in the ignition path and since it is exposed to the combustion process, it becomes a window into the engine without taking it apart. But spark plugs have a language of their own that is sadly known to only a few – however, this primer will change that. Understanding, reading and then employing proper service procedures to spark plugs is essential to get the most performance from any engine with an ignition system.


The spark plug is positioned in the combustion chamber at the point most suitable for igniting the compressed air/fuel mixture. It has to permit reliable cold starting, guarantee there is no misfire during any engine operating speed and load, and withstand operation for hours on end without a concern.

In some parts of the country, the spark plug can be as cold as minus 40 degrees F on a winter morning but as soon as combustion begins, the tip and electrode are exposed to a flame of around 1,000 degrees. A swing of 1,040 degrees almost instantly! The spark plug must be able to hold up to this thermal shock over tens of thousands of miles of operation.

The spark plug must also withstand mechanical demands that are created by the pressure in the cylinder, while still maintaining a gas tight seal.

If that were not enough, the part of the spark plug projecting into the combustion chamber is exposed to the high-temperature chemical process that takes place inside a cylinder during combustion. Components in the fuel may form chemically aggressive deposits on the spark plug, affecting its operating characteristics.

The deposits resulting from the combustion process, such as soot, carbon residues, and ash from the fuel and oil additives, will under certain conditions conduct electricity and cause the engine to misfire if allowed to build up.

The wear on the electrodes is caused by erosion and corrosion. This is defined as the burning away due to normal arcing, and by chemical and thermal attack.

The electrode gap is the shortest distance between the center electrode and the ground (side) electrode. The smaller the electrode gap, the lower the ignition voltage required to arc the plug.

A low voltage spark can transfer only minimal energy to the mixture, and ignition misfire can easily occur. Higher voltages are required to support an arc across a larger gap. This type of gap is effective in transferring energy to the mixture, but the associated reduction in ignition voltage reserve increases the risk of misfire at high engine speeds or cylinder pressures if the coil is weak.


The spark plug’s heat range is an index of its capacity to withstand thermal load. It must be adapted to the engine characteristics.

The varied designs of automotive engines with respect to operating load, compression ratio, engine speed, cooling and fuel make it impossible to run all engines with one standard spark plug. The same spark plug would get very hot (cause ping) in one engine but would reach only a relatively low average temperature (foul and misfire) in another.

To ensure the plug runs nether too hot or too cold in a given engine, plugs with different thermal capacities were developed. The so-called “heat range” is assigned to each spark plug, and is used to characterize these loading capacities. The heat range is a yardstick for selecting the correct spark plug. It is important to recognize there is no industry standard for heat range – it is determined by the spark plug manufacturer. Thus, a Champion spark plug for your engine may have a different heat range than an AC-Delco or Autolite for the same application.

Heat absorption is determined by the surface area of the insulator nose. If a large area is exposed to the combustion gases (this is achieved by having a long insulator nose), the insulator nose gets very hot. Conversely, with a short insulator nose the area is small and it remains cooler.

The heat range of a spark plug is identified by a code number. A low heat range indicates a “cold” plug with low heat absorption through its short insulator nose. A high heat range code applies to a “hot” plug with high heat absorption through its long insulator nose. But as mentioned previously, a cold plug in one brand may be a hot plug from another manufacturer.


It is important to keep the spark plugs in order when they are removed so the running condition of each cylinder can be examined.

For a traditional muscle/collector car engine, the proper spark plug color would be light tan with minimal degradation of the electrodes and gap. This plug can be put back into service and provide a long life.
  • A rich mixture will cause the entire plug to be coated with a black, powdered soot.
  • Excessive oil consumption will cause a black, gummy appearance on the spark plug. An engine with worn valve guides/seals that is running properly will have an insulator nose that is darkened on the side that faces the valve guide, while the rest of it will be clean.
  • Severe detonation (ping) will blister the insulator nose (a magnifying glass is required to see this), while a very lean condition will cause the plug to be extremely white.
  • Numerous cold starts without proper warm-up (especially with EFI) will place a clear coating on the spark plug that will cause it to misfire, and the engine to run rough. This coating cannot be removed, and the plug will need to be replaced.

Other than the cold start coating mentioned previously, a spark plug can be cleaned by placing it gently against a soft wire wheel on a bench grinder. It can then be re-gapped and installed. But if the engine running condition was extremely poor, new plugs should be installed after the issue is repaired.

It is important to note that other than the porcelain cracking from being dropped or improper handling, there is hardly anything that can go wrong with a plug. Spark plugs fail from an engine/fuel/ignition problem. They do not die – they are murdered.

You too can be a forensic spark plug investigator. Study this primer and you are well on your way to being an expert!

The threaded area of the plug is called the reach, and it helps determine where in the combustion chamber the electrode will reside. Tapered seat plugs have no sealing washer, while flat-seat versions do. The washer or gasket is always installed with the stepped side toward the cylinder head. The center electrode protrudes from the insulator nose, while the side electrode attaches to the exterior of the shell. Older engines used a plug shell with a 13/16-inch hex, while newer designs employ a 5/8-inch hex.

Dedicated spark plug sockets are required. Most plug sockets have a hex on the exterior so an open or box end wrench can be used where a ratchet will not fit. This is often the case when headers are installed.

A spark plug socket has an internal cushion that protects the porcelain insulator from damage when being installed.

Never grab the lead by the wire, always use the boot. It is best to twist the boot back and forth a few times to break it loose before trying to pull it off.

There are tools made to assist in pulling off a stubborn spark plug wire without breaking it.

Before removing the plug it is important to clean the area around the shell from any accumulated dirt so that it does not enter the bore and scratch the cylinder wall. Compressed air is best. Brake clean is an excellent substitute and usually won’t harm the engine paint. Then the plug can be removed.

Once the plug is out, use a spark plug-threaded chaser to clean out any carbon or debris. Plug threads are either 14 or 18mm. Many chasing tools are made with one dimension on each side. Always thread the chase in by hand. If a wrench is required use as little force as possible. If you reach a tight spot, back the chase out and then in a few times to clean it out instead of trying to force it. A chase differs from a tap since it is not designed to make threads in a blind hole, but rather, to clean them.

This is a perfect spark plug. The center electrode is sharp. The insulator nose, electrodes and shell perimeter are all clean. This engine has a perfectly adjusted carburetor.

The insulator and electrodes are clean, but carbon deposits are beginning to form on the shell perimeter. This engine is properly adjusted but was exposed to frequent cold starts without a complete warm-up.

Some gas deposits are beginning to form by the side electrode but more important, the insulator is starting to blister slightly from extreme pinging in this cylinder. The small divot in the cone may not be easily seen, but is there.

This engine is suffering from a very rich-running condition, along with deposits of ash from the composition of the gasoline. The rich condition is accelerating the deposit formation.

Here, the plug had a collision with a foreign object in the cylinder that crushed the side electrode and closed the gap.

This engine is tuned properly, but the plug is too cold for the application and is allowing excessive ash deposits to form. It will still run fine, but over time will misfire.

It may be hard to see, but the insulator nose by the side electrode is black while the rest is white. This engine has oil passing around either the valve guide or seal, but is still running fine.

Chemical additives in some gasoline can be very corrosive. This plug looks rusty but it is chemical corrosion.

Here’s a great example of a properly tuned engine with either too many cold starts, or a misadjusted choke/pull-off. The insulator nose is white and the perimeter of the shell is sooty black.

Bosch produces a spark plug with four side electrodes, but that does not change how you read it. The slight discoloration around the shell perimeter shows short trips but the engine is tuned properly. A good highway run of about 100 miles will completely clean this plug.

Ida Automotive markets a tool that easily measures the heat range of a spark plug. This is important for tuning a muscle car engine, since there is no industry standard for heat range. A hotter plug is more prone to detonation, while too cold a plug will foul easily. The tool retails for $69.95 plus shipping. Our sample plug was determined to be medium cold. A plug for the same application but by a different manufacturer measured to be medium hot. A potential problem for a high compression engine, regardless of what the auto parts counterman says.

When the proper plug is obtained, always start by checking that the side electrode is aligned perfectly over the center electrode. If not, use needle-nose pliers to gently tweak it into position. If any of the electrodes are completely closed, that plug was dropped at one time. Carefully check the porcelain for any cracks, which would lead to a misfire.

Many believe a round wire feeler gauge is the most accurate, while others prefer a flat gauge. The author has found no difference and suggests you use the style you find easiest to work with. It is important to hold the gauge straight and not angled. The measurement is accurate when a slight drag is felt on both the center and top electrode. If any adjustment needs to be made, gently pry or tap on the side electrode to obtain the desired gap.

Spray each new plug with carburetor cleaner while holding the center electrode toward the ground. This will remove any porcelain dust or metal shavings left over from manufacturing that can cause a misfire.

Carefully apply an anti-seize compound to the threads while keeping the rest of the plug clean. If any of the compound gets on the electrodes or porcelain, wipe it off. It is important to make the plug tight into the cylinder head for sealing and proper heat transfer into the water jacket. A loose spark plug will not only cause poor performance, but the possibility of detonation.

Before installing the secondary wire, put some dielectric compound into the boot. This will help eliminate corrosion, displace moisture and reduce sticking to the porcelain insulator. When finished, start the engine and it should run fine. It is possible to chip a piece of carbon from the combustion chamber while installing the new plug and have it short the electrode. If the engine has a dead cylinder with the new plugs, that is most likely the problem.


Ida Automotive Inc.
600 Texas Road
Morganville, NJ 07751


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