They also told us that their recommended torque setting can be misread by as much as 40 percent if the threads have been greased or lubed prior to installation. The danger is not so much that the plug will break, but rather that it will be very difficult to remove the next time around.
They pointed out another problem. Many anti-seize materials come in a jar with a brush built into the top. With such an applicator, it is very easy to put too much goop on the threads. When this happens, installing the plug will effectively pump the excess material either toward the spark gap end or the spark plug wire end of the spark plug. The likely story is that it will do some of both.
Anti-seize compound is basically a mixture of grease and metal particles. Aluminum, copper, and zinc powders are commonly used. The way this works is that whenever you have two different metals, in this case the aluminum of the head and the steel of the spark plug shell, you can set up a battery action if there is an electrolyte between them. This is called a galvanic cell. What happens is that you get a transfer of metal that could cause the plug to effectively weld itself to the cylinder head. Putting the metal particles between the threads of the plug and the threads of the head gives the galvanic cell something to work on rather than the materials of either the head or the plug shell.
Anti-seize works in the same way as putting zinc on a garbage can, which is called galvanizing. It prevents rusting. The zinc sacrifices itself to the rust reaction, leaving the steel protected. The downside to anti-seize is what happens when too much of it is used. The grease and the metal particles can wind up in places where it not only doesnt help, but it can actually interfere with the operation of the plug and the engine.
Several things could result from such contamination. The central electrode of the spark plug has a ceramic base that insulates it from the steel shell of the spark plug. To the extent that this is covered in metal particles and grease, it gives an unintended path for the spark. Instead of the spark being in the right place across the electrodes, it could wander off down the side of the plug. This can result in a misfire or even a late firing event.
To the extent that it is the grease that covers the firing end of the plug, this can cause a no-fire condition. Excess fuel fouling of the plug can cause much the same situation. When we look at these plugs, what we see is that we have deposits on the ceramic insulator that do not belong there. We can also see anti-seize residue on the threads and at both ends of the threaded area.