What would cause one of Shannon's spark plugs to shoot for low-earth orbit?

Dear Car Talk

Dear Car Talk | Jun 01, 2008

Dear Tom and Ray:

Yesterday I was driving my 1998 Honda Accord down the highway when I heard this horrible noise, like the tire blew. So I pulled over. All the tires were fine, but the car was still making a horrible noise. I lifted the hood and discovered that one of the spark plugs had blown right out of the engine, blown through the insulation and left a huge dent poking up in the hood. What would cause this? -- Shannon

TOM: What would cause this? More like, WHO would cause this? Whoever worked on your engine last, Shannon.

RAY: Funny things happen at garages. You're putting four spark plugs into a '98 Accord, and suddenly the roach coach shows up, and you get a craving for an anchovy, chipped beef, cheese and cholesterol bomb. So you go out to the truck, you chat with the other guys, you tell a few jokes and you eat your sandwich. Then you come back, pop the wires on the spark plugs and close the hood.

TOM: And, in your postprandial stupor, you forget to tighten the spark plugs. And since the explosions in the cylinders create a tremendous amount of pressure, the loose spark plugs immediately -- if slowly -- start working their way out of their holes. When one gets completely unscrewed, the pressure sends it flying upward, like the cork on a bottle of champagne.

RAY: So, the first thing you should do is check your other three spark plugs. There's at least a 75 percent chance that they all were left loose and you're soon going to have three more matching dents on your hood.

TOM: If they were just left loose, tighten them back up and you're all set. In that case, no damage was done except the dent.

RAY: The other possibility, though, is that whoever put in that spark plug over-tightened it or cross-threaded it and stripped the threads. That's a mistake a lot of rookie mechanics make. It means that the threads are ground down and there's really nothing to hold the spark plug in there. That's less likely to happen to all four plugs, but if a guy's capable of stripping one plug, he's capable of stripping more than one, so you'll want to check.

TOM: If the threads in the spark-plug hole are stripped, then you have to put in what's called an "insert," which is a metal sleeve with new threads for the spark plugs.

RAY: But whatever happened, it probably was caused by the last person who removed your spark plugs. If it happened to be a relative, Shannon, scratch him off this year's Christmas-card list.

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