1978 Ski-Doo Shop Manual, Page 267Get this manual
SECTION 05 SUB-SECTION 02, (SPARK PLUG)
BOSCH SPARK PLUG
BOSCH SPARK PLUG NUMBERING SYSTEM
Old system:
HEAT RANGE
The proper operating temperature or heat range of the spark plug is determined by the spark plug's ability to dissipate the heat generated by combustionThe longer the heat path between the electrode tip to the plug shell, the hotter the spark plug operating temperature will beand inversely, the shorter the heat path, the colder the operating temperature will beA "cold" type plug has relatively short insulator nose and transfers heat very rapidly into the cylinder headSuch plug is used in heavy duty or continuous high speed operation to avoid overheatingThe "hot" type plug has longer insulator nose and transfers heat more slowly away from its firing endIt runs hotter and burns off combustion deposits which might tend to foul the plug during prolonged idle or low speed operation
Indicates particular design detail