"Who's a Bright Spark then!"

The Spark Plug - a tiny little item in our motor bike engine, but oh so critical! Its primary purpose in life is to ignite the petrol and air mixture using high voltage electricity. But it has a secondary purpose also and that is, - it must also be capable of removing heat from this part of the engine.

So are they all the same then? 
Well basically yes, the construction of the spark plugs are very similar and except for material technology improving there has not been much in the way of changes for over 60 years - yes I did say 60 years!

It is very important however to make sure you fit the correct recommended plug type for each engine, as an incorrect plug fitment can at very least harm performance and cause breakdowns, and at worst destroy your motor!!!

So lets first look at some of the more obvious differences - 
Spark Plugs come in different lengths - classified as Long Reach and Short Reach Plugs.

14mm threaded plugs have been the most common fitment for over 40 years (thread diameter not spanner size), but some modern engines and especially four stroke motorcycle engines use plugs with very small thread diameters. 12mm is common on these and even 10mm on some specialist engines. In case your wondering why, - smaller plug sizes leave more room in the cylinder head for bigger valves and even 4and 5 valves, hence these engines can produce more power!!!

Spark plugs also come with different sealing methods. As the plugs themselves are a fairly hard metal, a sealing method is necessary to prevent high pressure combustion gases escaping past the threads. Many plugs use a soft crushable washer which can be obtained in a range of thicknesses, the other common method is a tapered seat plug (not common on motorcyccles yet), these do NOT need a sealing washer as they are designed to fit into cylinder heads with a tapered recess! ONLY fit the correct sealing type for your engine!

Besides the makers name, plugs come with all sorts of fascinating on them! The example shown here is taken from a typical NGK spark plug.

NGK - B P R 8 E S








1. The makers name.
2. Thread Diameter - B = 14mm.
3. Projected Insulator.
4. Resistor type plug.
5. NGK's Heat Rating.
6. Plug reach - E = 19mm.
7. Super wide range electrode.

Spark plugs are designed to work at the correct temperature for each engine. Too cold they will foul up easily, too hot and the plug tip will glow red and pre-ignite the fuel mixture, which can lead to engine destruction! For NGK a number 5 is a fairly hot running plug, designed to go into a lower performance engine. A NGK number 9 is a very cold running plug  which is designed to go into a highly tuned engine, it will conduct heat away from the electrode tip quickly so that it will not start to glow red hot.

These numbers unfortunately are different for each spark plug manufacturer. An NGK 9 is roughly equivalent to an ND 27. And Champion plug numbers run from hot to cold the opposite way around altogether, so never make an assumption or guess, - your engine could be at stake!

From the drawing here you can see where spark plugs are designed to lose their heat. It is essential that plugs are tightened correctly as nearly half of the heat is conducted away from the sealing washer area and if loose will cause the tip to overheat. 

In recent years many Speedway racing engines recently had troubles after they had their aluminium cylinder head fitted with steel thread insert for the spark plug, not realising that they would need to change their plug heat grade due to much poorer heat conduction!

It is interesting to note that although using different designs, most manufacturers claim their spark plugs are better than everyone else's! In the early 80's Bosch claimed a 2.5 horsepower advantage on a 'typical' car engine, it may have been true. I will only say that it would not be very difficult to set up a series of tests to suit almost any plug, which would prove that each of them are the best at something!! So you will have to draw your own conclusion on this.

Study these graphs below to see what really affects spark plug and hence engine performance.

Shown here are some typical graphs which display characteristic performance changes, relating to plug types, settings and other related conditions.

To prevent radio and TV interference all modern motor vehicles are fitted with resistive plugs (and leads). The law states that a total resistance of 10K ohms must be in the High Voltage circuit for each spark plug. This is often divided between the plug and the leads - at 5K ohms each and is normally designated by an 'R' in the plug coding.. Be warned, not using a resistive plug when one is specified can damage sensitive electronic equipment like fuel injection and electronic ignition units.

Reading spark plugs,  i.e. Studying their condition when removed, can prove an invaluable tool in diagnosing an engine.

  • These are typical signs to look for:

Engine detonation - causes peppered specks or cracks around the insulator nose.
Pre-Ignition - can also show peppered specks or melted electrode tips.
Fuel mixture analysis - White = weak mixture, Black = rich mixture, Rich tea biscuit colour = correct mixture.
Oil problems - If the engine is starting to burn oil, the plug will be covered in white powder. If the plug is shiny black, then it is oil fouling and will shortly fail.

Fine wire electrodes do provide better starting and performance on very highly tuned engines and will also resist associated fouling problems, but are a lot more money. Fitting new plugs regularly is definitely a benefit also, but always test them. I never race on an untested spark plug, I have made that mistake before and so have many others. Fitting a brand new set of duff plugs just before the a race start. Fitting colder plugs than required will lose you a small amount of horsepower and will foul up and need replacing far sooner.

Finally always use the manufacturers recommended torque settings, - this is critical and will prevent over tightening which can lead to thread stripping in the cylinder head, or even worse if loose on a racing engine, serious internal engine damage through failing to dissipate heat!

As a rough guide  - for 10mm, 12mm and 14mm spark plugs only if torque settings are not available, NGK suggest to screw the spark plug in finger tight until it is seated firmly home and then tighten 1/2 a turn for plugs with a gasket and 1/16 of a turn for taper seat plugs without a gasket. 

So what should the spark plug gap be? On a personal level I have always found that on my 2 stroke competition dirt bikes, plugs work best at between 22-24 thou, 0.55-0.6mm.

Well I hope you have enjoyed this, the first of a series of Technical Articles designed to help you get the most from your Dirtbikes.

Now get out there and have an enjoyable 'Dirty' month !!!

Until next time saludos - Adrian Harris

(NB: This article is offered as an item of interest only and Biker's Website, DirtyBiking and the author take no responsibility for any misinterpretation by any reader resulting harm or damage to either themselves or any equipment.)

Article Copyright   Adrian Harris 16/01/03. Not to be reproduced without prior permission.