A spark plug’s heat range describes its ability to dissipate heat, rather than spark intensity. In NGK’s numbering system, a higher heat-range number means a colder plug. The number is not a quality score: the plug must work within the thermal range intended for the engine.
Start with the motorcycle’s manual and the plug manufacturer’s application catalogue. Dimensions and firing-end projection must match as well as heat range; visual similarity is insufficient. Follow the vehicle manufacturer’s replacement interval and check the relevant guidance when changing to a different plug type.
Nickel, platinum and iridium electrodes also differ in wear resistance. Platinum and iridium can extend service life compared with conventional nickel plugs, but a more expensive material does not make an unsuitable component compatible with the motorcycle.
A standard engine normally needs the factory-recommended heat range. If a plug repeatedly fouls, investigate fuel delivery, oil consumption or another operating problem. Fitting a hotter plug can reduce the margin against pre-ignition, so it is not a universal remedy.