Spark Plug Heat Range - Ford Ranger Maintaince And Repair Manual

HOW TO USE THIS BOOK
Cross−section of a spark plug
Click to enlarge
A variety of tools and gauges are needed for spark plug service

SPARK PLUG HEAT RANGE

Spark plug heat range is the ability of the plug to dissipate heat. The longer the insulator (or the farther it
extends into the engine), the hotter the plug will operate; the shorter the insulator (the closer the electrode is to
the block's cooling passages) the cooler it will operate. A plug that absorbs little heat and remains too cool
will quickly accumulate deposits of oil and carbon since it is not hot enough to burn them off. This leads to
plug fouling and consequently to misfiring. A plug that absorbs too much heat will have no deposits but, due
to the excessive heat, the electrodes will burn away quickly and might possibly lead to preignition or other
78 Spark Plugs