To use most spark plug gap charts, measure the space between a spark plug's center electrode and its ground electrode to ensure the actual gap matches that listed on the chart, and make adjustments as necessary. Automotive parts stores carry a gapping tool that provides accurate measurements.
Spark plug gap charts reflect the ideal spacing for ignition under typical applications, and may not be appropriate for a particular use. Even factory pre-gapped spark plugs require measuring to ensure the spacing remains correct after shipping.
Narrow a too-wide gap by carefully tapping the plug's electrode on a hard surface. Widen the space using a gapping tool to pull up on the ground electrode without touching the center electrode or the porcelain. Avoid re-gapping multi-leg grounded plugs and racing plugs. Take special care with fine wire plugs, as the center electrodes easily snap, voiding any warranties.