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Whether you're shopping for new plugs, you just bought new plugs that you want to check before installing, or you want to check the function of your current set of spark plugs, you need to learn the appropriate gap between the two electrodes on the end of each plug.  Each vehicle's measurement will be different, though most are generally somewhere between 0.028-.06 inch. You can either look in your owner's manual or go to the auto parts store and have them look it up for you. Modified engines require smaller gap settings to account for the amount of power you're putting through the engine. General rule of thumb: the more power, the smaller the gap. There are several varieties of tools used to measure the gaps of spark plugs, some of which are more appropriate for more modern plugs, which sometimes feature delicate precious metals. Most measuring tools will also feature a flat edge typically used for bending the bottom electrode on the spark plug to adjust the gap slightly.  A coin-style gap measure is usually the cheapest option, and works by running the edge of the "coin" through the gap until you reach a point where it stops. The edge is marked like a ruler, designating the thickness of the edge at that point. This is a good tool for older plugs you want to check the efficacy of, but can also inadvertently widen the gaps as you use it. A wire coin gap works essentially the same way as the regular coin-style gap, but features different wire measures of varying lengths on the edge around the coin. A blade-style feeler gauge is an effective and versatile tool. Built like a pocket-knife, this gauge features different blades of varying widths, some with wire on the end and some without, placed in the gap to check the spacing of the electrodes. You can also use multiple blades to measure larger gaps. These are effective to use while adjusting. . If you've just pulled it out of the box, it should be in pretty good shape, but if you're checking a plug that you've been using in your car it's a good idea to clean it off a little with a clean cloth. Spark plugs can build up some whitish soot on the contact points, so you'll want to make sure they're somewhat clean to get the most accurate reading. You can use a little bit of quick-drying (90%) alcohol on the contact points to clean them off if they're especially dirty. Lots of build-up or any blackening on the contact points might be a sign of a plug that's outlasted its prime, however. If they're especially dirty, you might consider buying new ones. Place the appropriate blade or wire on your gap tool between the electrode tips of the plug, or pass the coin through the electrodes to determine the measurement.

Summary:
Learn your vehicle's gap specifications. Select an appropriate gap tool. Clean the spark plug Measure the gap by passing the tool through the electrodes.