KNOWLEDGE
We barely give them a second thought except when they break down, but these mini marvels of steel and ceramic have some clever secrets
Alan Seeley
‘I try to replace spark plugs before they fail, but only use the fancy ones when they were an OE fit. So my 2005 Fireblade gets expensive iridium, but my 1972 Commando gets old-style copper.’
The basics
A spark plug conducts a high-voltage (up to 45,000 volts or more) low-current pulse sent to the plug cap from an ignition coil. This then jumps the air gap from the centre electrode to a side electrode and ground (earth). A conventional side electrode covers the tip of the plug, though some leave the centre electrode more exposed to the fuel/air mixture.
The plug must work within the correct heat range for the engine it is specified for, usually between 400 to 800°C. Plug manufacturers cater for different engine types with a range of spark plugs featuring differing lengths of insulator nose. A long insulator maintains the temperature in the plug – these are called ‘hot’ or ‘soft’ plugs and are usually fitted in low-revving, low-performance engines.
A short insulator nose absorbs less heat and with more of the insulator in contact with the shell, heat is rapidly dissipated from the plug. These are termed ‘cold’ or ‘hard’ plugs and are usually found in high-revving, high-performance engines.
Modern plugs have a wide heat range, so you need good reason to deviate from the recommended fitment, but a slightly hotter plug may reduce fouling if most of your riding is bimbling in town, while a tuned engine may run better with a slightly colder plug.
Modern engines often use projected nose-style plugs in which the electrode and the end of the insulator extend a little further out of the plug. This allows quicker plug heating to reduce fouling, and also takes advantage of the cooling effect of incoming fresh fuel/air at higher revs. Plug manufacturers design their spark plugs for the type of engine your bike has, so this isn’t a variable you need be concerned with in plug selection.
What are the options?
COPPER SPARK PLUGS
In a copper spark plug, the central electrode is usually made from a heat and fuel-resistant nickel alloy with a copper core. Copper is a popular choice for plug construction, because it offers good conductivity of electricity plus the ability to dissipate heat. Copper also has the advantage of being cheap, making this type of plug an economical choice.
The more basic the engine, the more basic the plug required. A magneto engine will be happiest with a copper plug without a resistor to match the lower energy of the system.
+ Pros Cheap and easily available
- Cons Less resistant to erosion and not as forgiving of high temperatures
EXOTIC METALS
Higher-end metals such as silver, platinum, palladium and gold are sometimes used in the electrode alloy for their anti-erosion characteristics, or improved conductive properties. For example, gold is more resistant to corrosion and silver conducts better than copper. The iridium you often hear talked of in the context of spark plugs is a member of the platinum group of elements. They promise easier starting, greater efficiency and longer life. However, these benefits are really most tangible when they are used in high-revving, high-compression modern engines.
+ Pros Longer service life, better guarantee of spark efficiency
- Cons Expense
SURFACE DISCHARGE PLUGS
The spark of a surface discharge plug goes sideways from the centre electrode to the side electrode, which can be the metal shell itself. This gives the centre electrode unhindered exposure to the fuel/air mixture and theoretically keeps the electrode cleaner because the spark stops deposits forming on it. It is also intended to slow erosion by giving the spark several paths to earth, therefore keeping the side electrodes cleaner for longer.
These plugs are something of a niche and really only benefit hard-worked modern race engines. Probably not worth the expense for most of us.
+ Pros Long life and efficiency
- Cons Expense, limited range of fitments
Decoding a plug
The specification of a plug is indicated by the code on the insulator. The really important bits are heat range, thread size, reach and the material used in the plug’s core.
Typically, an NGK CR9E has a 10mm thread with a 16mm hex (C), a suppressor resistor (R), is 9 on the heat range (from 2 hot to 12 cold) and a reach of 19mm (E), which is the length of the thread. There may also be a letter denoting the firing end construction – for example a BR8ES has a standard copper core (S). This might be also be followed by a number which denotes the required gap in tenths of a millimetre (where there is no number, the gap is 0.7-0.8mm).
Different manufacturers use different codes and Champion’s heat range numbers work in the opposite way to NGK; high numbers are hot, low numbers are cool. Your bike shop and the internet have cross-reference charts if you want or need to deviate from the manufacturers OE fitting.
What are typical failures and when should plugs be replaced?
Given that a plug sparks thousands of times every minute, it’s not surprising that side and centre electrodes are prone to wear. The combination of extreme heat and vibration can damage insulators – and then there’s the constant attack from oil, carbon and combustion byproducts.
Older four-strokes that burn a bit of oil will foul plugs and, while they can be cleaned with a copper brush and brake cleaner, they will need more frequent replacement. Modern oil improves plug life on two-strokes, but they still need replacement more often than four-stroke plugs.
Check conventional copper plugs for gap and condition every 4000-5000 miles and replace at 8000-10,000 miles. Iridium plugs have longer inspection and replacement intervals of 15,000 and 30,000 miles or so. And only buy plugs from a reputable source; there are lots of low-quality fakes around.
Photography NGK, SIMON BELCHER/ALAMY & SHUTTERSTOCK/DEZAY