An NGK gap chart is an automotive reference that provides the specifications for the electrode gaps of spark plugs. The electrode gap is the distance between both of the spark plug's electrodes. This distance controls how powerful of a spark the plug generates.
For nearly all of the spark plugs that NGK manufactures, the spark plug gap is controlled only during the manufacturing process and is not adjustable by the spark plug's owner. However, certain spark plugs do have an adjustable plug gap for special applications. This is necessary for vehicles with high compression or a high fuel-to-air ratio. These vehicles require a greater voltage to ignite the fuel and therefore need a smaller spark plug gap.