The heat range of a spark plug is a measure of its ability to  transfer the heat of combustion to the cylinder  head. The  plug must operate hot enough to burn off carbon  deposits,  which can cause fouling, a condition where the plug no  longer produces a spark across the electrodes, yet remain  cool enough to prevent a preignition condition. Spark plug  preignition is caused by plug electrodes glowing red hot as a  glow plug, setting off the fuel-air mixture before the  normal  firing position. The length of the nose core is the principal  factor in establishing the plug’s heat range.  [Figure 4-34] Hot plugs have a long insulator nose that creates  long heat  transfer path; cold plugs have a relatively short i nsulator  to provide a rapid transfer of heat to the cylinde r head.  [Figure 4-34] If an engine were operated at only one speed, spark plug  design would be greatly simplified. Because flight  demands  impose different loads on the engine, spark plugs must be  designed to operate as hot as possible at slow speeds and light  loads, and as cool as possible at cruise and takeoff power. long, the plug may contact a piston or valve and damage  the engine. If the plug threads are too long, they extend into  the combustion chamber and carbon adheres to the threads  making it almost impossible to remove the plug. This can  also be a source of preignition. Heat of combustion can make  some of the carbon a source for ignition, which can ignite the  fuel-air mixture prematurely. It is very important to select  the approved spark plugs for the engine. The choice of spark plugs to be used in a specific  aircraft  engine is determined by the engine manufacturer after  extensive tests. When an engine is certificated to use hot  or cold spark plugs, the plug used is determined by the  compression ratio, the degree of supercharging, and  how the  engine is to be operated. High-compression engines tend to  use colder range plugs while low-compression engines tend  to use hot range plugs. A spark plug with the proper reach ensures that the  electrode end inside the cylinder is in the best position to achieve  ignition. The spark plug reach is the length of the threaded  portion that is inserted in the spark plug bushing of the  cylinder.  [Figure 4-35]  Spark plug seizure and/or  improper combustion within the cylinder can occur if a plug with the  wrong reach is used. In extrem cases, if the reach is too  An aircraft’s ignition system is the result of careful design and  thorough testing. The ignition system usually provides good,  dependable service, provided it is maintained and inspected  properly. However, difficulties can occur with normal  wear, which affects ignition system performance, especially  with magneto systems. Breakdown and deterioration of  insulating materials, breaker point wear, corrosion, bearing  and oil seal wear, and electrical connection problems are all  possible defects that can be associated with magneto-ignition  systems. The ignition timing requires precise adjustment and  painstaking care so that the following four conditions occur  at the same instant: If one of these conditions is out of synchronization with any  of the others, the ignition system is out of time. If the spark  is out of time, it is not delivered to the cylinder at the correct  time and engine performance decreases.  Figure 4-34.  Hot and cold spark plugs. Figure 4-35.  Spark plug reach. Reciprocating Engine Ignition System  Maintenance and Inspection 1.  The piston in the No. 1 cylinder must be in a position  a prescribed number of degrees before top dead center  on the compression stroke. 2.  The rotating magnet of the magneto must be in the  E-gap position. 3.  The breaker points must be just opening on the No. 1  cam lobe. 4.  The distributor finger must be aligned with the  electrode serving the No. 1 cylinder. 4-22 Hot Cold Reach