Spark Plugs Guide for Norton’s 1950-1975

Different manufacturers use specific coding systems to convey details about their spark plugs. Typically, the first digit indicates the thread size, followed by a number representing the plug’s heat range. Champion and NGK plugs generally align in this aspect, though compatibility with other brands may vary. This discrepancy arises because some manufacturers design plugs with broader heat ranges—one plug might cover the equivalent of two from another brand.

As a general guideline, for NGK and Denso, a higher number signifies a colder plug, whereas for Champion, a higher number indicates a hotter plug. The next letter usually denotes the plug’s reach, although Champion is an exception: its first letter indicates both the thread size and the reach.

Additional letters in the code describe special features, such as materials used or suitability for competition. For instance, a ‘P’ in the NGK range, a ‘Y’ in Champion, and a ‘D’ in Bosch signify that the plug has a projected electrode design.