® Spark plug facts

On Heat Ranges
Ever wonder why American spark plugs and those made elsewhere are numbered in opposite fashions? On Champion, AC and Autolite plugs a larger number indicates a hotter-running plug, while NGK, Denso and Bosch number theirs higher for cold-running plugs. Why the difference? It has to do with perspective. American engineers view the plug largely as a heat producer, that is, they focus on the plug's heat additiverole. Thus a larger number is hotter. The other manufacturers see the plug as a heat remover and focus on the spark plug's role of maintaining a certain amount of heat and getting rid of the rest. To them a larger number is a plug that is more of a heat sink than is a smaller number, Both views are valid, obviously. They're just different.

When we talk about "hotter" and "colder" we are talking only of the plug's ability to retain enough heat to keep itself clean. We are not talking about the spark. A spark plug has to fight off deposits and it does so by burning them off. The amount of heat necessary to do this is pretty much the same from one plug to another. What changes is how much heat is transferred from the plug--the hottest point in the engine--to the rest of the engine. Thus a hotter plug is one whose heat conductivity is slower (through a longer heat path) while a colder plug is one whose conductivity is faster (a shorter heat path). The spark plug is properly matched to the engine when it retains just the amount of heat it needs to ward off carbon deposits, and no more. In fact not only is more heat useless, it is harmful as it can quickly lead to plug overheating and hence, preignition. So when someone says a plug has a "hotter spark," this is colloquialism for a larger spark, or a higher voltage spark, and has nothing to do with the plug's heat range, which is a different thing altogether.

Servicing Plugs
Folks have more than a little trouble with spark plug maintenance. They remove them from the engine and then check spark, putting themselves in danger as this is a prime way to burn your bike down, not to mention your garage and house! Cross-threading plugs is another quick route to anguish, not the least because even after you get the plug out, you still have to repair the threaded hole in the cylinder head, and there is no easy way to do that. Some people overtighten their spark plugs, again ruining the theads in the engine or making the plugs difficult to remove. Proper plug torque is very low, about the same as for a 6mm bolt. It may seem odd to go this low, but remember plugs are mostly hollow, and modern plugs in particular are quite thin on their walls. Many current model powersports vehicles take tiny 10mm spark plugs whose body's wall thickness is deceptively narrow. These plugs have to come back out eventually, of course, and overtightening them will cause them to snap off at the threads because a plug, like any bolt, requires more torque to remove than to install. In connection with this, few powersports techs are fans of putting anti-seize compound on spark plugs. None of the major OEMs recommend it.

Elsewhere I have written about reading spark plugs, but suffice to say at this point it is not the science many believe it is, at least under average conditions. Another forum fallacy. And once you have sand-blasted plugs, you can forget all about reading them. In fact, plugs should never be sand-blasted, despite what many schools continue to teach. Not only does sand-blasting make carbon adhere easier, it also changes the plug's heat range, is too aggressive on modern plugs' delicate fine wire electrodes, and the abrasive can and will stick inside the plug's cavities to come out later inside the cylinder. All around not a good practice.

Related reading
Resistor Plugs, Wires, and Related Issues
U-Gap and Split-Fire Spark Plugs
Powersports Ignition Evolution
High Performance Ignition Coils
GL1000 Ignition Tech
Reading Spark Plugs
Optimizing Early SOHC Four Point Ignition
Servicing the CBX Ignition System
Honda CBX and DOHC Ignition System Troublshooting
The CBX Ignition Trilogy
Tuning Up the Honda SOHC Four
Plug Wire Tech
Spark Plugs

Last updated April 2023
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