Spark plugs only fulfill one purpose in life – they deliver energy produced by the coil to the combustion chamber in the shape of a spark. However, when this is 40,000 volts at a temperature of over 500-degrees, they have to be built out of strong stuff! A spark plug has a shell, which is the part you screw into the engine, and within this shell is the ceramic. The ceramic is the insulator that keeps the high voltage that is passing through the centre of the spark plug from earthing – if you have ever touched an uncovered HT lead when the engine is running you will know why this is necessary! In the centre of the plug you have a terminal, which is the bit you connect to the coil via the plug cap, then a resistor seal and a central electrode, which is connected to the tip - the bit that sparks. Finally, you have the ground electrode, which the spark jumps to from the central electrode and earths the whole spark plug through the plug’s body and then the bike itself. Both electrodes are made from nickel alloy for strength and then plated in precious metals such as iridium to increase life span and improve performance.