1978 Polaris RXL Service Manual, Page 27Get this manual
SPARK PLUG HEAT RANGE
Th term "heat range" refers to the classification of spark plu gs accordi ng to their ability to transfer heat from th firin tip of the insulator to th cyl inder headThe rate heat transfer is co ro lled basically by th dist ance the heat must tr avel to reach the cylinder
Since eac engine has different temperature characterist ics, yp ical empe ra tur readings with hot and co ld running plugs are illustra tedA 'cold' type plug has relat ively short insu lator no se and ransfers heat very rapidly into he cyli nder head Such plu is used in heavy duty co nti nuous high speed operat ion to avoid overheating