Spark Plugs for Nortons

The codes used by the various manufacturers give some information about the plug types. The first digit represents the thread size, followed by a number which reflects the heat range of the plug. The Champion and NGK plugs match well in this respect, but do not necessarily match well with others. This is because some have wider heat ranges and one plug may cover two from another manufacturer. As a rule of thumb, for NGK and Denso, a higher number means a colder plug whereas for Champion, a higher number means a hotter plug. The next letter describes the reach of the plug, except in the case of Champion where the first letter describes not only the thread but also the reach. Any other letters cover special features or materials used or if the plug is for competition use. A 'P' in the NGK range, a 'Y' in the Champion, or a 'D' in the Bosch range means that the plug is a projected electrode type.