Spark plugs operate at extreme temperatures, electrical  pressures, and very high cylinder pressures. A cylinder of an  engine operating at 2,100 rpm must produce approximately  17 separate and distinct high-voltage sparks that bridge the  air gap of a single spark plug each second. This would appear  as a continuous spark across the spark plug electrodes at  temperatures of over 3,000 °F. At the same time, the spark  plug is subjected to gas pressures as high as 2,000 pounds  per square inch (psi) and electrical pressure as high as 20,000  volts. Given the extremes that spark plugs must operate under,  and the fact that the engine loses power if one spark does  not occur correctly, proper function of a spark plug in the  operation of the engine is imperative.  The three main components of a spark plug are the electrode,  insulator, and outer shell.  [Figure 4-32]  The outer shell,  threaded to fit into the cylinder, is usually made of finely  machined steel and is often plated to prevent corrosion from  engine gases and possible thread seizure. Close-tolerance  screw threads and a copper gasket prevent cylinder gas  pressure from escaping around the plug. Pressure that might  escape through the plug is retained by inner seals between the  outer metal shell and the insulator, and between the insulator  and the center electrode assembly. The other end is threaded  to receive the ignition lead from the magneto. All-weather  plugs form a seal between the lead and the plug that is water  proof to prevent moisture from entering this connection.  The insulator provides a protective core around the electrode.  In addition to affording electrical insulation, the ceramic  insulator core also transfers heat from the ceramic tip, or nose,  to the cylinder. The insulator is made from aluminum oxide  ceramic having excellent dielectric strength, high mechanical  strength, and thermal conductivity. The types of spark plugs  used in different engines vary in respect to heat range, reach,  massive electrode, fine wire electrode (Iridium/platinum),  or other characteristics of the installation requirements for  different engines. The electrodes can be of several designs from massive  electrodes or Nickel-base alloy to fine wire electrodes.  [Figure 4-32  and  4-33]  The massive electrode material has  a lower melting point and is more susceptible to corrosion.  The main differences include cost and length of service.  Fine wire iridium and platinum electrodes have a very high  melting point and are considered precious metals. Therefore,  the cost of this type of spark plug is higher, but they have  a longer service life with increased performance. Fine wire  spark plugs are more effective than massive electrode plugs  because the size shields its own spark from some of the fuel  air mixture. Less than efficient combustion occurs due to  uneven ignition. The iridium electrode allows for a larger  spark gap, which creates a more intense spark that increases  Figure 4-32.  Spark plug cutaway.  Figure 4-33.  Fine wire electrodes.  performance. The spark gap of any electrode is vulnerable  to erosion and the melting point of the electrode material. 4-21 Outer shell Electrodes Insulator 5 /8 " -24 Shielding  barrel with connector 3 /4 " -20 Shielding  barrel with connector