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Spark plugs operate in severe conditions. Spark plugs require regular maintenance to provide desired engine performance. The service life of a spark plug is directly related to the voltage required to produce a spark in the electrode gap. As the voltage required to produce a spark increases, the service interval is reduced and the overall service life decreases. Higher voltage reduces the service life of components such as spark plugs, wires, and transformers.

The following six main factors affect the voltage required for spark plugs:

Air-fuel ratio – When the air-fuel ratio is near stoichiometric, the voltage required to produce a spark is the lowest. As the air-fuel ratio becomes leaner, the voltage required becomes higher.

Compression ratio – As compression pressure increases, the required voltage increases.

Ignition timing – As ignition timing is retarded, the voltage required to produce a spark will increase.

Electrode wear – As the spark plug electrode wears and becomes more rounded, the voltage required to produce a spark will increase.

Fuel BTU – Lower BTU fuel reduces the spark plug service life.

Electrode gap – The wider the spark plug gap, the more voltage is needed to produce a spark.

All of the factors above, except for compression, can be adjusted somewhat on most gas engines. As compression increases, the required voltage increases. A cleaned and regapped plug requires more voltage than a new plug. As the gap increases on a plug the voltage required for spark increases dramatically. The required voltage can exceed the arc over voltage of the plug. Arc over voltage is the amount of voltage required to produce a spark between the electrode and the ground strap. Fuels that cause high levels of deposits can cause compression to rise well beyond normal. Higher compression increases the voltage required closer to the arc over voltage.

The service life of the spark plug varies for different applications. The service life can be affected by various factors and operating conditions. 

  • Remove a spark plug by hand after the initial torque is broken.

  • If the spark plug resists removal by hand, apply penetrating oil to the threads. To help the oil penetrate the threads, turn the spark plug back and forth until the spark plug is loose.

  • If the spark plug could not be removed by hand, clean the threads with a spark plug seat cleaner. The seat cleaner scrapes debris from the seat and from the threads in the cylinder head. Be sure to remove any debris from the cylinder.
  • After removing the spark plug, remove the crushable metal spark plug gasket.

  • Always replace crushable spark plug gaskets when installing or reinstalling a spark plug.

  • Orient the tabs of the crushable gasket toward the spark plug electrode. Otherwise, the gasket may not seat properly.

  • If a gasket for a spark plug is installed incorrectly, do not increase the torque on the spark plug in order to improve the seal.
  • Always regap the spark plug when the spark plug has been removed.

  • Always use a new gasket when installing the spark plug.

  • Install the spark plug into the seat by hand. If the spark plug cannot be seated by hand, there is a problem with the threads.

  • Always finish installing a spark plug with a torque wrench and spark plug socket.


Note: Normal operation will produce a light ash deposit on the portion of the plug exposed to combustion. Heavy ash deposits are a sign of poor combustion or high oil consumption. Clean such deposits from the plug.

  • Inspect the spark plugs closely to look for damage and/or to determine if the ignition system is operating correctly.

  • Check the entire plug for mechanical damage, such as cracked insulators or metal cases. If any cracks are found, replace the plug.

  • Faint marks may extend from the top edge of the metal case onto the insulator. The marks may be the result of corona that forms at the top of the metal case. Corona develops when a high voltage potential ionizes the air that surrounds a conductor. The marks are a normal condition. The marks are not a result of leakage between the metal case and the insulator. Many times these marks are misinterpreted as combustion leaks and the plugs are discarded with many more service hours available.

  • Look for eroded electrodes. An eroded plug is a worn out plug. Spark plugs in this condition will require more voltage to produce a spark. A worn plug can require more voltage than the ignition system can supply.

Spark Plug Cleaning

  • Solvents can be used to clean oil and tar deposits from all plug surfaces with no adverse affects on the plug.

  • When used with caution, glass beads can be used to clean the combustion end of the spark plug. Apply the glass beading only to the electrode area of the spark plug. Do not apply glass beads to the case or to the threads. Clean the threads with solvent and a clean wipe. Do not reuse the glass beads.

  • Always keep the ignition components clean and free of dirt and oil.


Note: A spark plug can operate despite a buildup of ash. Large deposits may retain heat that can cause premature fuel ignition. Large deposits can lead to uncontrollable detonation.

  • Inspect the spark plug pre-combustion chamber for deposits.

  • Make sure that the holes in the spark plug pre-combustion chamber are not blocked.

  • A light brown deposit or a beige deposit is produced by normal operation. Deposits that are gray or black may be caused by the following conditions: excessive oil, use of the wrong oil, a substance that is introduced through the fuel system or the air system and poor combustion because of a rich air/fuel mixture.

  • Condensation can form in spark plugs that have pre-combustion chambers. Condensation can cause difficulty for cold start-ups. If an engine shuts down during the warm-up cycle, condensation can form. The condensation can cause start-up difficulty until the condensation evaporates or until the plugs are changed. Engine block heaters will help avoid this situation.

Spark Plug Cleaning

  • Do not clean and reuse spark plugs with pre-combustion chambers. If the buildup of ash is considerable, change the plug.


Note: If you change all of the spark plugs from J-gap plugs to plugs with pre-combustion chambers, the air fuel ratio may need adjustment. The engine can be run leaner with plugs that have pre-combustion chambers.

  • Do not clean the spark plug with a wire brush. Deposits of brass on ceramic can cause the spark plug to short circuit.

  • Do not use a thread tap to clean spark plug threads in the head. A tap is designed to remove metal. This practice will eventually result in thread failure in the cylinder head.

  • Do not mix types of transformers in an engine.

  • Do not mix types of spark plugs in an engine.

  • Do not mix spark plugs with pre-combustion chambers and J-gap spark plugs in the same engine.


  • Do not use anti-seize compound on spark plugs. Most of the heat is transferred through the threads and the seat area of the spark plug. Contact of the metal surfaces must be maintained in order to provide the heat transfer that is required.

  • If a gasket for a spark plug is installed incorrectly, do not increase the torque on the spark plug in order to improve the seal.

  • Do not over tighten a spark plug. Over tightening the spark plug can cause cracks. Over tightening can also yield the metal, which loosens the shell. Discard any spark plug that has a shell that is cracked or loose.