Issue under which the core nose
of a spark plug is likely to fracture:
1. Mishandling of the insulator/center electrode (CE) during assembly. Prior to final assembly, the insulators, CEs and shells of spark plugs are handled as bulk parts, meaning that they are placed in bins for the purposes of storage and transport. While insulators are exceptionally strong (if you want to try something interesting place an old plug in a lathe, turn part of the shell away, remove the insulator and try to break it with a hammer- be sure to wear glasses/goggles), on rare occasion handling them in this manner will cause one to fracture slightly. Often this fracture doesn’t become evident until the plug is run in an engine. I should emphasize that this is so rare that you don’t really have to take it into consideration in the real world.
2. Improper gapping procedure. While the insulator in general is very strong, core nose (the part of the insulator visible inside the shell) doesn’t tolerate lateral loading very well. What happens is that the circular “ramp” style of gap gauge that’s popular today is often placed between the CE and ground electrode (GE) and used to wedge the electrodes apart until the correct gap is achieved. The problem with this method is that it places a lateral load on the core nose that can easily cause a fracture. Almost invariably this fracture won’t be visible to the unaided eye and often can’t be seen with a magnifier. However, after the plug is run a few laps, the fracture will get larger and ultimately a section of the core nose will split off. If the split is large enough the plug will cease firing, or only fire intermittently. Worse, a large piece of the core nose is more likely to get between the piston and head, possibly causing damage to both when the piston is at or near TDC. It can also land between a valve and seat, when it can damage either one or both, not to mention what it can do to rings. If you’re lucky, it will go out the exhaust without touching anything.
The proper way to gap a plug is to either use a small screwdriver or, preferably, the electrode bending tool that’s usually part of your gap gauge, to open or close the GE. The GE bending tool should be used to adjust the gap, then the ramp should be inserted and used to check the gap, not widen it by force. The best and most accurate type of gap gauge is the round plastic model incorporating wire loops of various wire sizes and a GE bending tool. These work particularly well with corner-gap plugs. You can use this tool to bend the GE and the wire loops to check the gap size without fear of laterally loading the core nose. One final note: if you use a screwdriver or similar tool to bend the GE, be absolutely sure not to make contact with either the CE or the core nose, as damage to the plug may result.
If your plugs with broken core noses don’t exhibit any signs of detonation and/or pre-ignition, but rather look like normal plugs in terms of the combustion process, then it’s very likely that the breakage is the result of an improper gapping procedure.
3. Excessive torque on installation. When spark plugs are assembled, the shell is held upright and a thin copper gasket is placed inside the shell, where it rests on a shoulder near the bottom of the shell. The insulator is lowered onto this gasket, the another copper gasket is placed on a shoulder of the insulator near the top part of the shell, then finally the top part of the shell is “turned over”, locking the shell and insulator together (this is oversimplified but illustrates the construction of the plug). The important thing to note is that the copper gaskets, particularly the lower one, are there to help dissipate the heat of combustion. When the engine is running, the core nose and CE obviously are very hot. This heat travels down the core nose, though the lower copper gasket to the shell/head/coolant, where it is dissipated.
Proper heat dissipation depends upon a solid contact between the copper gasket and the insulator. When a plug is installed with the correct torque, this contact is normally very good and the heat will transfer and dissipate normally. When a plug is over torqued, though, things go downhill quickly. When the proper torque is applied, the threaded portion of the shell will stretch just enough to provide an adequate clamping force to keep the plug tight. However, when the plug is over torqued, excessive shell stretch can cause the lower copper gasket to move away from the insulator. If this happens the heat of combustion can no longer dissipate normally. The core nose and CE will become excessively hot, hot enough to cause pre-ignition. Usually, though, the core nose will split from the intense heat and the plug will quit firing before pre-ignition occurs.
4. Insufficient torque on installation. In this scenario the plug itself functions normally but the threads make very poor contact with the threads in the head, greatly impairing heat transfer. This can cause the core nose and CE to become very hot but the biggest problem is that hot gases can start to leak between the threads on the shell and the threads on the head. If this happens the leak becomes progressively worse and part of the threads in the head can literally be burned away. If undetected, this can ruin a head pretty quickly.
5. Detonation. The extreme cylinder pressures that accompany detonation can literally break things, including the core nose of a spark plug. These pressures can also break pistons, resulting in pieces of piston and ring that can impact the core nose and break it. If you see a fractured core nose and other evidence of impact and/or extreme heat, at the very least you’d better take a close look at your pistons, valves, head, etc. for damage.
6. Preignition. Preignition, which can either
be caused by detonation or can occur on it’s own, tends to melt things.
If preignition is occurring the heat generated is far beyond what the core
nose and CE can safely dissipate. If you’re lucky the core nose will split
from the intense heat and the plug will quit firing. If have seen plugs
where the core nose got so hot that it was actually physically distorted.
Of course, the CE and GE are usually long gone by this point. Since the
melting point of most any ceramic is far higher than the melting point
of aluminum or iron, if you see this kind of damage on one or more spark
plugs then you can assume that the piston and valve(s) are history, too.