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.