Spark plug gaps are measured with a spark plug gap gauge that consists of varying thicknesses of metal used to slide between the terminal and the insulated tip of the spark plug. There is a specific tool for use on spark plugs, but a standard metal feeler gauge works just as well. After properly measuring the gap and reading the gauge users need to consult the car owner's manual to find the recommended gap settings for their specific vehicle.
Spark plug gap gauges and feeler gauges are available at most auto parts stores. Typically, new spark plugs come pre-gapped from the manufacturer and need no additional adjustment before installation. However, occasionally rough handling in shipping can change the gap, so it is a good idea to check it. Thoroughly clean old spark plugs before measuring to ensure an accurate reading.
The gap gauge consists of several wires of varying thicknesses with each marked as to its measurement. Choose one and slide it through the gap. If it does not touch both the terminal and the tip it is too thin. Select a larger wire and repeat the process until you find one that touches both the tip and terminal but still passes through. If the wire doesn't pass through, it is too thick and you must go down in size.