spark plug gap

The spark plug gap is vital to burning the fuel mixture efficiently. When you buy plugs for your bike, they usually come with the correct gap between the electrodes – I haven’t seen one wrong for about 15 years. Iridium plugs have a very fine, and easily damaged centre electrode, so long-gone are the days of levering the gap open with a feeler gauge.

Manufacturers recommend the use of a wire spark plug plug gap tool to prevent damage if you do want to check or adjust them. However, to confirm that the gaps are correct, I still use my old feeler guage, being careful not to put any pressure on the delicate centre electrode.


We spoke to NGK to find out what the official line on a spark plug gap is:

“On older vehicles there were fewer part numbers of plugs produced, so gap setting used to be a common practice to ensure the spark plug gap was set suitable for each machine.

With modern machines, spark plugs have become much more application specific, which is one of the reasons why we now have more than 800 individual part numbers. One part number may now only cover 6 or 7 entries, compared to older listings when a part number may cover 30-40 models.

A good example of this is our current car spark plug catalogue; more than 50% of the entries have no gap information published, as the plug listed is pre-set for the intended vehicle. In the motorcycle catalogue, we still publish gap setting for every line, but in most cases the recommended item is manufactured with this gap pre-set, as it contains over 230 unique spark plug part numbers.
The plugs that we recommend for the engine are usually those that have the gap setting that matches the recommendation of the vehicle manufacturer.


But, more importantly, some of the designs are such that it is impossible to change the gap, especially with multi-ground electrode spark plugs: In trying to change the spark plug gap it alters the geometry of the firing end, so it is definitely not recommended to do so. Fine wire plugs must be treated with great care, and again we would not recommend altering the gap setting.

For more information about NGK Spark Plugs (UK) Ltd and its products and services, please visit the website at

Tony Carter

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