What Causes the Center Electrode to Crack on a Spark Plug?

Spark plugs are ubiquitous enough to be taken for granted, and simple enough in appearance to belie their inherent weaknesses and hidden complexities. Cracked plug insulators aren't the most common problem out there, at least not for most, but they do indicate some problem in need of attention in the engine.

Parts of a Spark Plug

  • Most spark plugs consist of three parts. Starting from the inside out, a spark plug starts off as a piece of very thick metal wire extending from the top cap where the plug-wire connects through to the bottom tip of a spark plug. A ceramic insulator about 1/8-inch thick surrounds the center electrode wire, and keeps it isolated from the outer metal body. When positive current passes through the center wire, it jumps a small gap between the wire tip and a ground strap tab attached to the metal body of the plug, which itself grounds to the cylinder head.

The Center Insulator

  • When you look down at the tip of a spark plug, you'll notice that there's actually a small gap or channel around the white ceramic insulator and the outer body of the plug. The ceramic used in a spark plug is a good electrical insulator, but pretty poor at insulating against heat, which means that it readily absorbs and transfers combustion heat away from the metal tip and into the cylinder head via the plug body. The white part you see cracked isn't the metal tip, it's the ceramic insulator around the electrode.

Plug Heat Ranges

  • You may wonder why there's a groove around the ceramic insulator, why it doesn't just extend all the way to the tip of the electrode. The center electrode needs a certain amount of heat to efficiently produce a spark; in practical terms, the hotter the better up to the point that the electrode melts. The groove serves as a tuning aid to reduce or increase heat transfer from the plug to the head. A deeper groove means a hotter-running plug and higher heat range, and a shallower groove means a colder plug and a lower heat range.

Heat Damage

  • Ceramic is pretty sturdy stuff from a thermal standpoint, but it can only take but so much stress before it cracks. Three things will cause heat damage: an excessively lean fuel mixture, very high combustion chamber temperatures, and pressures resulting from the use of a turbo, supercharger, nitrous or high-compression pistons, and using too "hot" of a plug for the engine. This is a crucial aspect of tuning high-performance engines, where the act of choosing the proper heat-range plug is often a matter of trial and error.

Pre-Ignition and Detonation

  • The ceramic used in spark plugs is just like most any other ceramic in that it doesn't respond well to hammer-like shock loads. Engine knock, ping, pre-ignition or detonation -- call them whatever you want, but they all send shockwaves bouncing around the cylinder. Those powerful waves have about the effect you'd expect them to have on shatter-prone ceramic, and create a tell-tale sort of damage to the insulator and electrode. Whereas heat-induced damage will often cause one or more tiny hairline cracks in the insulator, detonation damage will break off large chunks of the insulator.

Other Damage

  • Oddly enough, damage during manufacture, shipping and insulation are quite common where cracked insulators are concerned. When installing and torquing the plugs, mechanics have a tendency to forget that they're dealing with what amounts to a ring of glass in a metal shell. Dropping the plug on a concrete floor and over-torquing it during installation will break the insulator, as will under-torquing the plug. The latter will break the insulator by impairing heat transfer into the heat, thus concentrating energy on the insulator and causing it to run hotter than it should. Water leaking into the cylinder will also crack the plug, shattering the ceramic by causing it to rapidly cool and contract.