Repairing a spark plug that broke off flush with its hole.

Dear Car Talk

Dear Car Talk | Aug 01, 2004

Dear Tom and Ray:

Yesterday, my son broke off a spark plug in his 1989 Ford F-150 with a six-cylinder engine. It broke off flush with the hole. Egad! How do we get the rest of the spark plug out, and what other concerns must be kept in mind? -- Dave

TOM: Well, it depends on how he broke it, Dave. If he broke it by turning the wrench the wrong way (trying to tighten it instead of loosen it), then you might be in luck.

RAY: In that case, you would use an extractor -- something like Easy-Out -- which taps into the remaining piece of spark plug and grabs onto it with sharp, fluted edges. Once the extractor is attached, you simply put your socket wrench on the end of it, unscrew it in the proper direction, and the whole thing should walk right out.

TOM: If, on the other hand, it broke because it was rusted and fused in there, and he had the 7-foot ratchet extension on it and was walking around the car trying to unscrew it, the job gets a lot harder.

RAY: If the plug broke despite the fact that you were unscrewing it the right way, then you have to drill it out. This is a job best left to professionals, Dave. It's not for the faint of heart -- or the faint of talent. A mistake here can cost you a cylinder head.

TOM: What a mechanic will do is drill out the broken piece of spark plug, along with the existing threads. The result will be a smooth hole that will be bigger than the old spark-plug hole. He'll then use a tool to cut new threads for an insert, and the spark plug will go into that insert. Then you'll be all set.

RAY: He'll also have to fish out all of the metal shavings that fall into the cylinder during this operation. We get most of them with a magnet. If you don't remove them, they could cause serious engine damage.

TOM: So, you have to start, Dave, by holding your kid's feet to the fire and finding out exactly what he was doing when the spark plug broke. He'll be embarrassed to admit it if he was turning the wrench the wrong way, but that will actually make his life a whole lot easier.

RAY: This is one of those rare cases where stupidity pays off.

TOM: Aha! I've been looking for cases like that.

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