Why do spark plugs last longer nowadays? First of all, because of the use of unleaded fuel, then also because of the materials being used. Earlier, it was normal practice to measure the electrode gap and to re-adjust it by bending the ground electrode. This is now a thing of the past. At the moment, the ground electrodes are mounted in such a way that they cannot be bent nearer to the center electrode.
Just imagine if something was to break off while trying to adjust the gap, or even worse, something breaks off while the engine is running. Rather keep your hands off the spark plugs. As a matter of fact, the plugs should be taken out and screwed in again as seldom as possible. Since not only in the aluminum cylinder head, the threading is subject to a certain amount of wear and tear.
In the actual sparking process, a distinction is made between air-gap- and surface to electrode sparking. If the electrode gap is the shortest distance between the center- and the earth electrode, then as a rule, the air-gap spark takes this path exactly. Surface-to-electrode plugs prefer the surfaces, even those of the insulators, before the spark finds its way to earth. This often has the effect of removing impurities from the insulator.
The working temperatures of the spark plug also play a role as far as wear and tear of the electrodes is concerned. 850°C is considered to be the upper limit. It is astonishing that the spark plug passes up to 80 percent of the heat it takes in, to the cylinder head and only the rest to the effluent gas. What is important here, is the thread, and above all, its length. This also applies for the durability of the plug, and even moreso, that it is screwed in using a torque wrench.
Damage to the plug itself is very difficult to see. Fine cracks can often only be seen under a microscope. Indeed, who would take the trouble to do this, instead of changing them directly? Thereby, one can see a problem e.g., in the mixture formation or the engine mechanics if the plug is oiled- or sooted up. Apart from that, any deposits should also be examined. In modern engines it's not that easy to recognize that the plugs are running too hot. Earlier this was indicated if the electrodes were a whitish colour, instead of red-brown. 06/12