Spark plug FAQ’s

A common misconception is that a hotter spark plug will make and engine run hotter – this is not the case. Engine temperature is governed by factors such as the mixture setting (carburation) and ignition timing. The engine determines the temperature of the spark plug and not the other way round! The ideal situation is to match the engine and spark plug to ensure that the spark plug tip runs hot enough to prevent fouling, but stays cool enough to prevent overheating and subsequent damage to the engine.

For more information about spark plug heat ranges, including various photos and illustration that will help explain the above more clearly, please refer to my spark plug temperature guide.

A common misconception is that a cooler spark plug will make and engine run cooler – this is not the case. Engine temperature is governed by factors such as the mixture setting (carburation) and ignition timing. The engine determines the temperature of the spark plug and not the other way round! The ideal situation is to match the engine and spark plug to ensure that the spark plug tip runs hot enough to prevent fouling, but stays cool enough to prevent overheating and subsequent damage to the engine.

For more information about spark plug heat ranges, including various photos and illustration that will help explain the above more clearly, please refer to my spark plug temperature guide.

The majority of vehicles can run quite happily on resistor spark plugs, and in fact, most manufacturers recommend and fit them as standard. Some modern cars even now apparently require that resistor plugs are fitted as the engine management systems use the feedback from the plugs as part of the on-board diagnostics system.

However, classic motorcycles do not tend to have such a powerful ignition system as modern cars, especially those that are fitted with magneto type ignition. The extra effort and voltage required to make a spark pass through the resistor as well as jump the spark plug gap may just be too much causing weak, intermittent or complete loss of a spark. Obviously this isn’t good, hence non-resistor type spark plugs are recommended for such older vehicles.

You can usually spot a resistor spark plug from its manufacturers code. For example, NGK resistor spark plugs have a letter “R” near the start of the plug code, for example a BR7ES or BPR6HS.

Some spark plug caps also feature an in-built resistor for exactly the same reason and are also generally unsuitable for older vehicles. Fitting both a resistor plug cap and a resistor plug is completely unnecessary and double-trouble for classic machines with twice the resistance for the ignition system to overcome.

For more information about spark plugs see here.

The advantage of this is that these precious metals are stronger and less reactive than copper and so the central electrode can be made much finer. A standard copper electrode is a couple of millimetres across, whereas the diameter of a gold-palladium alloy electrode is only 1.0mm, a platinum electrode is 0.8mm and an iridium electrode can be made down to 0.6mm.

Having a finer centre electrode means that the voltage required for the spark to jump across the spark plug gap is reduced as the charge is more concentrated at the tip. This can potentially benefit classic bikes (especially those with magneto ignition systems) by making it easier for the engine to fire a spark. They should also last longer than a standard plug making it necessary to change plugs less often.

However, the verdict is still out whether these ‘performance’ spark plugs are worth the extra expense and results will probably vary from bike-to-bike and owner-to-owner. One thing to watch out for though is that some of these plugs now only come in resistor types (see other FAQ’s for what this means) which are generally unsuitable for classic motorcycles.

In order to optimise any engine, the manufacturer will spend months (or years) of time and effort conducting tests and research. The spark plug cannot be considered in isolation since it affects (and is also affected by) other engine parameters such as fuelling (carburettor settings), ignition (timing), fuel quality, expected running conditions and many more. In 99.9% of cases the manufacturer probably knows best and so we should stick with what they say.

There are a few exceptions to this rule though. The first is where a particular type of spark plug is not available because it has been discontinued or because of what is sold where you live. For example, in the case of my 1951 Matchless motorbike, the manufacturer specified a KLG FE80 spark plug. Not so easy to find now! So we must try to work out what the ‘modern’ or current equivalent plug is for our particular application. In this case the spark plug manufacturers catalogues can come in handy as they often contain equivalence table, both between different manufacturers and from discontinued models. For classic vehicles, speaking to the owners club or other enthusiasts is probably the easiest way forward as someone else has probably already come across the same dilemma and found a solution.

Another exception for when the manufacturers spark plug recommendation might not be optimum is where the engine has been changed from its original specification, for example when an engine is tuned for racing. If you know enough to tune an engine, you’ll already know enough to select teh right spark plug so I won’t go into that here.

For the larger sized 18mm plug thread the correct tightening torque is 35 to 40 Nm and for the smaller 12mm thread it is 15 to 20 Nm.

In all cases though, the spark plug should be initially tightened by hand in order to ensure that the thread is not damaged. Alternatively a length of rubber tube may be slipped over the end of the plug and this used to insert and initially tighten it up where additional reach is required. Inserting a spark plug using a socket to start with is likely to result in thread damage as it is more difficult to feel whether the thread is tightening correctly or threading. It is difficult to apply too much force resulting in thread stripping when tightening by hand.

An alternative to using a torque wrench is to tighten the plug as tight as possible by hand and then tighten it an extra 1/2 to 2/3 of a turn (180 to 230°) using a socket to squash the washer. When reusing spark plugs, the washer will already have been squashed by during the original fitting and so the plug should only be tightened an extra 1/12 of a turn (30°) beyond hand-tight using a socket.

Flash-over is when the spark jumps from the terminal at the end of the spark plug (where the HT lead connects) to the metal body of the lower part of the plug. The ribs or corrugations in the outside of insulator increase the surface distance between the terminal and metal body, thereby making it harder for a voltage to ‘flash-over’ the outside of the plug.