⚡Spark plug

How it works

The spark plug is the component that triggers combustion in a gasoline engine. It receives a high-voltage pulse from the coil (typically in the range of 5,000 to 30,000 V depending on the system) and turns it into a spark that jumps between its center electrode and its ground electrode, at the precise moment in the cycle when the air-fuel mixture is compressed. This spark ignites the mixture and starts the power stroke that drives the piston. The plug works under extreme conditions: intense heat, high pressure, thousands of ignitions per minute. Its operating temperature has to stay within an ideal range — not too cold (fouling, deposits), not too hot (pre-ignition, melted electrode): that's the role of its "heat range," which must match the engine, hence the importance of sticking to the original reference. The gap between the electrodes (often around 0.6 to 1.1 mm depending on the model — check your manual) determines the quality of the spark. Failure modes: fouling (a dry black deposit if the mixture is too rich, an oily deposit if oil is getting into the combustion chamber), normal wear (rounded electrodes, a widening gap, a weaker spark), flooding (a plug soaked in fuel after repeated start attempts), and cracking of the ceramic insulator. On a two-stroke, the plug fouls faster because of the oil in the fuel mixture. Typical signs of a worn-out plug: hard starting, misfires (especially under load and at high revs), power loss, higher fuel consumption, rough idle. The look of a removed plug is an excellent indicator of engine health: a light tan electrode signals healthy combustion.

Maintenance tips

• ✓Check the plug's condition and gap at every service, or every 10,000 to 20,000 km depending on your riding and the type of plug; on a 2-stroke, check more often since fouling happens faster. Check the exact interval in your manual.
• ✓Measure the gap with a feeler gauge and adjust it to the manufacturer's spec by gently bending the side electrode (never the center one). A correct gap ensures a strong, clean spark.
• ✓Read the electrode's color: light tan means healthy combustion; black and dry means too rich a mixture or a dirty filter; black and oily means internal wear; white means too lean a mixture, with a risk of overheating.
• ✓Respect the torque spec (often a quarter turn past gasket contact for a flat-seat plug, or the value from your manual). Overtightening strips the threads in the cylinder head — a costly repair.
• ✓Always fit a plug of the reference and heat range recommended by the manufacturer (trusted brands like NGK, Denso). Don't change heat range on a whim.
• ✓Keep a spare new plug of the correct reference under the seat: it's a cheap part that can get you out of trouble roadside in case of flooding or fouling.
• ✓Apply a thin trace of copper anti-seize to the thread if the manufacturer allows it, to make future removal easier, especially on aluminum cylinder heads.