Originally posted by Big Jim:
Detonation does not break the external portion of the insulator.
Detonation can break the inner portion of the insulator, the cone, that is exposed to the combustion chamber.
The external part of the insulator is broken by mechanical damage, usually during installation.
Sometimes (but very rare) a poorly made spark plug may blow apart so that the ceramic portion slips out of the matal shell. In my experience, this was more common with Champion plugs, but I have seen it on others. Although extreme cylinder pressure (including spikes in pressure from detonation) can contribute to this, it is mostly a problem with building the plug.
I was told this also, and it makes sense, but, I've had 6 plugs, 2 Autolites and 4 Motorcrafts with broken insulators. Always in the same place, on the top part of the ceramic insulator, above the built in hex nut used for installation. I was accused of breaking the plugs by the dealer, so I had the dealer install them, a week later I had 2 broken plugs. So I know it wasn't me, and it wasn't defective plugs. So something else is breaking them, but what ? How can a bad plug wire break the insulator ? Since I replaced my plug wires I have had no plugs go bad. Before I pulled the plug wires off to R&R my original plugs I never had a plug go bad. So I am just trying to envision how the insulators break, and I just can't. I'm hoping someone here who is smarter than I (that won't take much) can explain to me how it is that this happens, thank you.
Ken, 99 Silver Frost SE Zetec Atx, polished alloys, Kumho's, Pioneer hi power CD/AM/FM. Factory spoiler (added) Audi side lites, Borla stainless steel muff. Stolen, recovered in pieces. Looking for another Tour, preferably V6, 5 speed.
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