A glow plug is a thermal device used in methanol,nitromethane-powered internal combustion model engines. It provides the necessary heat for igniting the fuel/air mixture.
Similar in execution to glow plugs used to help start some full-sized diesel engines, R/C glow plugs are first energized with the aid of an electric device called an ignitor which clips to the top of the glow plug. Most ignitors come with a factory-installed nickel cadmium sub-C-cell battery for power while some inexpensive ignitors utilize an ordinary C-cell alkaline battery. Regardless of the type used, the ignitor forces current through the glow plug's platinum-based heating element, causing it to glow yellow- to white-hot, hence the name. Once the engine is started and running properly on its own, the ignitor is removed. From there, the element retains heat from each combustion cycle of the engine through a catalytic reaction with the methanol in the fuel, continuing to glow until the engine is powered down.
Glow plugs, much like spark plugs in gasoline engines, wear out in time but are easily replaced simply by unscrewing them from the cylinder head with a 7mm socket wrench or T-handled glow plug wrench and carefully screwing in a new plug. Gapping a glow plug is unnecessary. Another comparison to spark plugs involves heat range. Variables such as fuel, ambient temperature and general atmospheric conditions may require the use of a glow plug of either a cooler or hotter heat range than what came installed on the engine at the factory.