Always check that the spark plug gap is compatible with the engine manufacturers specification. A gap that is too small means that the spark duration will be very quick and the spark will be thin and weak. The consequences of this may be bad starting and high exhaust emission levels. This will result in an increase in fuel consumption. If the gap is set too large, the ignition system will not be able to cope with the demands and a misfire situation will occur. Some wide gap spark plugs have a longer ground electrode to accommodate a wide gap setting. These must be used where specified, as opening up a standard plug to a wider gap setting may result in the electrodes not running parallel to each other. This could result in abnormal and premature electrode wear.

The re-gapping of fine wire spark plugs that have very small diameter platinum or iridium alloy electrodes is not recommended due to the risk of damaging the electrodes. Virtually all NGK spark plugs are set to the correct gap for the catalogue applications at the point of manufacture.