Spark plugs quit working primarily because of deposits routine cleaning can't dislodge. The ceramic insulator around the center electrode goes a long way up into the steel plug shell before it makes contact with the shell. The further the heat from the tip has to travel before dissipating itself into the cylinder head, the "hotter" a plus is said to be. On cold and medium-range plugs, carbon deposits from either rich fuel soot or carbonized oil can provide a path to ground for the firing voltage instead of making it jump the gap to the ground electrode and provide a spark to ignite the fuel/air mixture. Back in the dark ages, we used a dedicated sandblaster to clean plugs, one that blew fine pumice abrasive material all the way to the inner shell of the plug to dislodge deposits. Many years later, I used a glass bead blaster to clean race car plugs between races. I had 3 or 4 sets of spare plugs that we cleaned that way, and swapped them out while adjusting fuel mixture during practice. Those 4 sets of spark plugs would last the entire racing season, with just a cleaning between uses.
An important part of spark plug servicing is keeping the center electrode filed off square. A squared-off electrode requires about 15% less voltage to jump a spark than a rounded tip does, regardless of the plug gap involved.
Jerry
