Indexed spark plugs align the open end of the electrode toward the intake port. The hottest flame meets the new fuel charge for a performance gain, increases fuel mileage and plug life. This is a speed secret car racers have used for years. You simply mark the electrode side of the plug with a magic marker on the porcelain. You then shim the plugs with the washers to align the electrodes toward the intake ports. Fits 14mm threaded spark plugs.
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