Figure 4-55.  Spark plug cleaner tester. Spark plug Heli-Coil inserts are cleaned with a round wire  brush, preferably one having a diameter slightly larger than  the diameter of the spark plug hole. A brush considerably  larger than the hole may cause removal of material from  the Heli-Coil proper or from the cylinder head surrounding  the insert. Also, the brush should not disintegrate with use,  allowing wire bristles to fall into the cylinder. Clean the insert  by carefully rotating the wire brush with a power tool. When  using the power brush, be careful that no material is removed  from the spark plug gasket seating surface, since this may  cause a change in the spark plug’s heat range, combustion  leakage, and eventual cylinder damage. Never clean the Heli- Coil inserts with a cleaning tap, since permanent damage to  the insert results. If a Heli-Coil insert is damaged as a result  of normal operation or while cleaning it, replace it according  to the applicable manufacturer’s instructions. Using a lint-free rag and cleaning solvent, wipe the spark  plug gasket seating surface of the cylinder to eliminate the  possibility of dirt or grease being accidentally deposited on  the spark plug electrodes at the time of installation. Before the new or reconditioned plugs are installed, they must  be inspected for each of the following conditions: then the spark gap should be set using a round thickness  gauge. The spark plug should be tested by using a tester as  shown in  Figure 4-55 , which passes a high-voltage through  the spark plug and fire the gap. As this test takes place, the  firing end of the plug is subjected to air pressure to simulate  the pressure in the engine’s cylinder. If the firing pattern  is good, the plug should be returned to its holder ready for  installation in the engine.  Before installing new or reconditioned spark plugs in the  engine cylinders, clean the spark plug bushings or Heli- Coil inserts. Brass or stainless steel spark plug bushings are usually  cleaned with a spark plug bushing cleanout tap. Before  inserting the cleanout tap in the spark plug hole, fill the flutes  of the tap, or channels between threads, with clean grease to  prevent hard carbon or other material removed by the tap from  dropping into the inside of the cylinder. Align the tap with the  bushing threads by sight where possible, and start the tap by  hand until there is no possibility of it being cross-threaded in  the bushing. To start the tap on installations where the spark  plug hole is located deeper than can be reached by a clenched  hand, it may be necessary to use a short length of hose slipped  over the square end of the tap to act as an extension. When  screwing the tap into the bushing, be sure that the full tap  cutting thread reaches the bottom thread of the bushing. This  removes carbon deposits from the bushing threads without  removing bushing metal, unless the pitch diameter of the  threads has contracted as the result of shrinkage or some  other unusual condition. Replace the cylinder if, during the  thread-cleaning process, the bushing is found to be loose,  loosened in the cylinder, or the threads are cross-threaded  or otherwise seriously damaged. The gap setting should be checked with a round wire- thickness gauge.  [Figure 4-56]  A flat-type gauge gives an  incorrect clearance indication because the massive ground  electrodes are contoured to the shape of the round center  electrode. When using the wire thickness gauge, insert the  gauge in each gap parallel to the centerline of the center  electrode. If the gauge is tilted slightly, the indication is  4-34 5.  Install a new spark plug gasket. When the thermocouple  gasket is used, do not use an additional gasket. 4.  Inspect the inside of the shielding barrel for cracks in  the barrel insulator, and the center electrode contact  for rust and foreign material that might cause poor  electrical contact. 3.  Check both ends of the plug for nicked or cracked  threads and any indication of cracks in the nose  insulator.  2.  Check for evidence of rust-preventive compound  on the spark plug exterior and core insulator and on  the inside of the shielding barrel. Rust-preventive  compound accumulations are removed by washing  the plug with a brush and cleaning solvent. It must  then be dried with a dry air blast. 1.  Ensure that the plug is of the approved type,  as indicated by the applicable manufacturer’s  instructions. Inspection Prior to Installation