Spark plug

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Spark plug with single-ground electrode.
Iridium tipped spark plug that is standard on the Honda VFR800

A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels inside of a combustion chamber. Simple in design, spark plugs consist of only five to seven parts. The design is really ingenious. Every feature of the plug performs several critical functions. For example, the ceramic insulator (the white part of the plug) not only keeps the spark from grounding out before reaching the spark gap, it prevents Resistor flashover and helps control the heat range of the plug.

Spark plugs are engineered to match the engine exactly. If the plug is too short, the electrode will not be deep enough into the combustion chamber, resulting in poor idle and uneven running. If the plug is too long, the plug may be struck by the piston or valves. Heat range refers to the temperature of the electrode when at operating temperature. A "cold" plug is a plug that dissipates heat faster than a "hot" Shell plug. A colder plug may be needed if the engine is burdened for a long time under a full load. A hotter plug may be needed if the engine is run for long periods at partial throttle. Changing to a colder or hotter plug must be done with great care. If the plug runs too hot, preignition will result. It is Electrode also possible to damage the pistons and valves when the plug runs too hot. If the plug runs cold, deposits will build up, resulting in fouling.

Some spark plugs come with a resistor element. This element reduces radio interference originating in the motorcycle ignition system. NGK adds a "R" to the end number to designate a resistor plug. If the bike has lots of radio equipment, or if it has electronic ignition, use a resistor plug.

There are many variants of spark plugs, designed to improve performance such as iridium tipped spark plugs and V-tipped spark plugs designed to increase the size of the spark for more reliable combustion.

Spark plugs are often thrown away needlessly. Unless the plug is dead, the insulation cracked or the electrodes burned away, the plug can be cleaned, regapped and used again.


Check the following and replace if necessary.

  • Insulator for damage
  • Electrodes for wear
  • burning condition, coloration;
    • dark to light brown shows good condition.
    • excessive lightness shows faulty ignition timing or lean mixture.

Reusing a Spark Plug

Clean the spark plug electrodes with a wire brush or special plug cleaner.

Check the gap between the center and side electrodes with a wire-type feeler gauge. If the gap is not as specified, bend the side electrode to adjust it.

Replace the spark plug in the cylinder head and hand tighten.Torque to specification.