Each engine generates its own individual vibrations. The higher the engine’s revving range and output, the stronger is the vibration.
Since the spark plugs are fitted in the engine’s cylinder head, they are subject to the same vibrations as the engine. As shown in Figure 1, the tip of the ground electrode vibrates with the weld as the fulcrum. If the ground electrode’s weld becomes unable to withstand the vibration fatigue, it will break.
See Photo 1.
Note also that if the spark plug is insufficiently tightened when fitted (i.e., the torque was less than the standard tightening torque), the plug may loosen. If it does loosen, it will be subject to abnormal vibration and this can cause the ground electrode to break off.
Note 1: The standard spark plug types fitted in high-revving, high-output engines are the multi-electrode type that is not so susceptible to electrode breakage and the type with a sparking portion that does not project very far; be sure to use the authorized spark plugs.
