Spark Plug

High voltage from the ignition coil jumps the gap between the plug’s electrodes, creating a spark. Plugs have a defined heat range and electrode material; long-life plugs with platinum or iridium tips last much longer than nickel ones. Plugs wear gradually: the gap grows and the coil needs more voltage to fire it. Replacement intervals are in the service schedule.

Plugs differ in thread, reach, heat range and electrode material. Iridium and platinum plugs last much longer than basic nickel plugs. Reading old spark plugs can give useful clues. A light grey or tan colour is normal; black, sooty plugs suggest a rich mixture or misfires; oily deposits can mean oil entering the cylinder; and white, blistered insulators point to overheating. Plugs should be changed on a cool engine to avoid damaging the threads.

The spark lights a small flame kernel that spreads through the mixture. The plug’s insulator also carries heat away to the cylinder head. The high voltage from the coil jumps the gap between the centre and ground electrodes, creating a spark that ignites the compressed mixture.

Worn plugs cause misfires, hesitation under load and hard starting. An oily or sooty plug can show that a cylinder is burning oil or running rich. Worn plugs cause misfires, hard starting and higher fuel use. Replace them at the interval in the service schedule and tighten them to the specified torque.