So just how does a car spit out a spark plug? Part 2.

Currently my Neon is at my local dealer under going an intense examination. There were about a third of the threads left in the spark plug hole. I have been maintaining the spark plugs, along with brakes, oil and many other things as a backyard mechanic for five years. Unfortunately I have to work on the street, I'm an apartment dweller. So the work I can perform is limited. The first time I looked at the spark plugs on this car was 1.5 years after I had purchased it. I drove the Neon almost 64 000Km before I finally got around to looking at the plugs. They weren't loose or damaged. I cleaned them, re-gapped them and put them back in. Seven months and 25 000Km I cleaned them and re-gapped them again. Nine months and 33 000Km I repeated the ritual again. One year and 36 000Km later I decided that maybe I should replace the spark plug, after all they had been in the car for at least four years and 163 494Km. I installed the Bosch Platinum plugs with the understanding that they where better and would last longer than stock copper plugs. That was my second mistake, the first being that I bought these plugs instead of the copper plugs. The platinum plugs have been in the car for just over a year and about 54 000Km. Unfortunately I wasn’t aware that you had to pay more attention to platinum plugs than you would if you used copper plugs, i.e. 1.5 years before I even looked at the stock copper plugs and they were still tight. After driving for almost four years and over 163 000Km with the copper plugs, not once did I have a problem with the ignition system. Since then, in the span of just over a year and more than 54 000Km I have had one plug physically ejected from the engine and three more that had backed off from their torqued position. Is it possible that I over tightened the #3 plug? Sure. But my experience is that when you over tighten something you usually know it. As you torque the screw it will get harder to tighten until it wants to stop. If you continue to exert force you will exceed the strength of the material and strip the threads. You would know this immediately because the screw would become easier to turn. This never happened with the plug. Could it have been cross threaded? It’s not very likely. It would have been impossible to thread the plug into the cylinder head by hand, at the end of a rubber hose, and overcome the strength of the material. My guess would be that you would need at least a force in excess of 20 foot/pounds to damage the aluminium head. That’s a lot to ask the average unaided hand to exert.

My belief is that the platinum plug has a tendency to back off from its torqued position and therefore must be regularly re-torqued. My failure to realize this, and not inspect the plugs on a regular basis has resulted in this incident. This unfortunately doesn’t explain why there are still some threads left at the bottom of the cylinder head.