erosion rate of steel or any of its known alloys, spark plug  manufacturers are using tungsten or an alloy of nickel for  their massive electrode plugs and iridium/platinum plating  for their fine wire electrode plugs. applying the lubricant to make certain that smeared fingers,  shop towels, or brushes do not contact the electrodes or any  part of the ignition system except the spark plug threads.  Never apply to the first set of threads. Erosion of the electrodes takes place in all aircraft spark  plugs as the spark jumps the air gap between the electrodes.  [Figure 4-50] Spark plugs should be removed for inspection and servicing  at the intervals recommended by the manufacturer. Since the  rate of gap erosion varies with different operating conditions,  engine models, and type of spark plug, engine malfunction  traceable to faulty spark plugs may occur before the regular  servicing interval is reached. Normally, in such cases, only  the faulty plugs are replaced. The spark carries with it a portion of the electrode, part of  which is deposited on the other electrode. The remainder  is blown off in the combustion chamber. As the airgap  is enlarged by erosion, the resistance that the spark must  overcome in jumping the air gap also increases. This means  that the magneto must produce a higher voltage to overcome  the higher resistance. With higher voltages in the ignition  system, a greater tendency exists for the spark to discharge  at some weak insulation point in the ignition system. Since  the resistance of an air gap also increases as the pressure  in the engine cylinder increases, a double danger exists at  takeoff and during sudden acceleration with enlarged airgaps.  Insulation breakdown, premature flashover, and carbon  tracking result in misfiring of the spark plug and go hand in  hand with excessive spark plug gap. Wide gap settings also  raise the coming in speed of a magneto and therefore cause  hard starting. Since spark plugs can be easily damaged, careful handling  of the used and replacement plugs during installation  and removal of spark plugs from an engine cannot be  overemphasized. To prevent damage, spark plugs should  always be handled individually and new and reconditioned  plugs should be stored in separate cartons. A common method  of storage is illustrated in  Figure 4-51 . This is a drilled tray,  which prevents the plugs from bumping against one another  and damaging the fragile insulators and threads. Additionally,  the tray helps identify where the spark plug came from and  its proper rotation. If a plug is dropped on the floor or other  hard surface, it should not be installed in an engine, since  the shock of impact usually causes small, invisible cracks  in the insulators. A dropped spark plug should be discarded. Spark plug manufacturers have partially overcome the  problem of gap erosion by using a hermetically sealed resistor  in the center electrode of spark plugs. This added resistance  in the high-tension circuit reduces the peak current at the  instant of firing. This reduced current flow helps prevent  metal disintegration in the electrodes. Also, due to the high  Before a spark plug can be removed, the ignition harness lead  must be disconnected. Using the special spark plug coupling  elbow wrench, loosen and remove the spark plug to elbow  coupling nut from the spark plug. Take care to pull the lead  straight out and in line with the centerline of the plug barrel.  If a side load is applied, damage to the barrel insulator and  the ceramic lead terminal may result.  [Figure 4-52]  If the  lead cannot be removed easily in this manner, the neoprene  Figure 4-51.  Spark plug tray. Figure 4-50.  Spark plug gap erosion.  4-32 Spark Plug Removal Gap Erosion of Spark Plugs