Harley-Davidson sparkplugs (Figure 5H-1) have been designed to give maximum life and efficient combustion of fuel. They are available in various "heat ranges," each for a particular service application. Plugs are labeled with numbers 2, 3, 4, or 5 the lowest number indicating the "hottest" plug. Designations 3-4 and 7 are special-purpose plugs.

For normal service, the spark plug as recommended in motorcycle specifications, Section 1-A, should be used on a particular model. However, for special service conditions, a "colder" or "hotter" plug may be desired. The number 2 plug, for example, may be best for slow speed operation while the number 5 plug would be best for the higher speeds of highway travel or special high-speed operation. The intermediate plugs (3 and 4) are designed for use in motorcycles with moderate or average service applications. It is not uncommon for best results to be obtained with plugs of different heat ranges in front and rear cylinders, with the front usually the colder.

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