Everything you need to know about spark plugs 

No petrol engine is without spark plugs: For your engine, a constant and sufficiently powerful ignition spark is essential for good cold starting, high performance, low emissions and economical fuel consumption.

Depending on the engine speed, a spark plug has to deliver an ignition spark to the engine between 500 and approx. 5,000 times every minute. At the same time, it operates under widely fluctuating thermal loads and large pressure differences – it is therefore one of the wearing parts on a motorcycle that is subjected to extreme stresses, and must be changed at regular intervals, as specified by the manufacturer. 

At the top of the spark plug is the terminal (Fig. 1a) for attaching the spark plug connector. Here, the high voltage of the ignition coil from the ignition unit is fed via the thick HT (high tension) lead to the spark plug. The terminal (SAE cap or 4 mm thread) must always be compatible with the spark plug cap used in order to ensure a secure fit!

The centre electrode (Fig. 1 b) conducts the current to the other end of the spark plug, from where it jumps as a spark to the ground electrode (Fig. 1 C), thereby igniting the fuel-air mix in the combustion chamber.

An interference-suppression resistor (Fig. 1 d) in the centre electrode prevents radio frequency interference, or RFI, (e.g. so that the spark plug does not interfere with your radio reception) and, at the same time, protects your motorcycle's sensitive electrical system against electromagnetic pulses. If you use a non-resistor spark plug, then the spark plug cap must have a resistor instead. To ensure that the voltage is only discharged where it should, the centre electrode and resistor are sheathed in a ceramic insulator (Fig. 1 e). The ribbing of this insulator also provides flashover (leakage current) protection (Fig. 1 f) by lengthening the path which stray electrons have to travel.

This is how an intact spark plug looks. The whitish-grey discolouration is normal. It comes from fuel additives that are not fully combusted, and is the result of controlled, normal combustion.

Here you see a spark plug with heavy deposits. These may be caused by, for example, poor fuel quality, high oil consumption in a mechanically worn engine or combustion of coolant due to a damaged cylinder-head gasket. They increase the likelihood of autoignition (the deposits continue to glow).

A broken insulator, as shown in Fig. 4, can lead to engine damage. The breakage is usually caused by using an incorrect tightening torque or if spark plugs are dropped on a hard surface (e.g. your garage floor) before they are fitted.

On this spark plug, the centre and ground electrodes have fused together. This happens when the spark plug overheats. In this case, there is a possibility that the piston will also have melted. The cause could be choosing the wrong spark plug (incorrect heat range) or an engine malfunction (knocking or autoignition).

Carbon build-up occurs if the spark plug is frequently operated at below its self-cleaning temperature (450°C) – if, for example, you only go short distances, or if you choose an incorrect heat range (too cold).

For many motorcycle models, technically superior iridium spark plugs can be used as an alternative to the standard version. They have an iridium alloy on the tip of the centre electrode. Iridium is a noble metal, and one of the hardest metals in the world. It only melts at 2,450°C or above, and is very resistant to spark erosion. For this reason, the life of iridium spark plugs is double that of standard spark plugs! What's more, the noble metal allows the centre electrode, at 0.6 mm, to be much thinner.

This, in turn, considerably reduces the level of ignition voltage required, makes the ignition spark more powerful, and helps to improve the expansion of the flame front in the combustion chamber. So iridium spark plugs not only have a longer life, but also optimise combustion in the engine. The electrodes themselves self-clean more effectively, so the spark is less susceptible to carbon fouling. What this means is improved starting, engine performance, spontaneous throttle response and fuel consumption. Iridium spark plugs are also ideal for classic motorbikes, or if you do a lot of town riding.

Either the spark plug or the cap must have a resistor to prevent the spark from causing radio interference and to protect your motorcycle's electrical system. This is a statutory requirement. On a motorcycle, it is sufficient for the spark plug or cap to be fitted with a 5 kOhm suppression resistor. If the resistance is too high, it will weaken the ignition spark, So it is neither necessary nor advisable to use a suppressed spark plug and suppressed cap together (although most engines would still fire).

The correct gap between ground electrode and centre electrode is decisive for a strong ignition spark. If the gap is not correct, the engine may misfire or your fuel consumption may rise. If the ground electrode is bent in relation to the centre electrode, e.g. because you accidentally dropped the spark plug on the ground, it will not fire. If the gap is too small, the combustion would be poor, and if the gap is too big, the ignition energy required is greater than the ignition system can deliver.

As a rule, the gap is between 0.6 and 0.8 mm, depending on the motorcycle model. Gaps of up to 1.1 mm are less common. You will need to check the table provided by the spark plug manufacturer or the information from the motorcycle maker, and then measure the actual gap on the spark plug using a feeler gauge. If the gap is not right, you can bend the electrode very carefully with a small screwdriver or pliers. You should give the spark plug a good clean with a special wire brush before installing it.

You should always let the engine cool down before changing the spark plug, to avoid damaging the thread. Pull the cap off the spark plug and examine it first of all for damage and corrosion (change it if necessary). BEFORE unscrewing the spark plug, examine the spark plug recess and remove any dirt (e.g. with a vacuum cleaner). Unscrew and remove the old spark plug using the correct size of spark plug wrench.

It is recommended to lubricate the spark plug thread lightly with copper or ceramic paste (not lubricating grease) before installing it. This will prevent the spark plug from seizing due to overheating, but has the disadvantage that dirt and grains of sand can very easily stick to the thread if you unscrew the spark plug and lie it down. This would result in damage to the thread – so it's important to keep everything very clean when you're doing anything with the spark plugs.

If you do not have a torque wrench, you can follow the old rule of thumb instead: If the spark plug is new, screw it in by hand, and then tighten it carefully by a half-turn with the spark plug wrench so that the gasket seats itself and the spark plug is secure. If you are fitting a used spark plug on which the gasket has already seated itself, you should only tighten by a quarter-turn.