What could cause spark plugs to blow out?

Dear Car Talk

Dear Car Talk | Jun 01, 1999

Dear Tom and Ray:

My wife owns a 1995 Ford Contour. About three months ago, while driving, she began to hear a loud clacking noise. After inspecting the engine, I found that a spark
plug had been completely ejected from the head. The threads on the head were completely stripped out. Then it happened again, only on another spark plug. Could it be
that there is just too much compression in the engine and this is causing the spark plugs to be expelled? If so, what can be done? -- Fred

RAY: It's definitely not a compression problem, Fred. No engine has enough power to blow out spark plugs. Certainly not a '95 Contour.

TOM: It's an unusual problem, though. I've had spark plugs blow out on me before, but it's always been when I forgot to tighten them. And when that happens, they just
slowly work their way out and then blow--with a frighteningly loud sound. But that never strips the threads. You just screw the plug back in and everything's fine
(except for the dent on the underside of the hood made by the "launching" spark plug).

RAY: I have occasionally seen plugs that were mismanufactured -- where the threaded part of the plug was slightly too small. In that case, it might not blow out right
away, but might leave room for hot exhaust gases to erode the threads over time until there were no threads left. But the odds of that happening on two different
cylinders are about the same as the odds of Bob Dole become a national spokesman for male impotence. All right, bad example. But trust me, it's VERY unlikely.

TOM: More likely, last time you were in for service, some rookie mechanic cross threaded a couple of your plugs. That means the threads on the plugs were lined up
incorrectly with the threads in the cylinder head. And when he tried to screw in the plugs, he damaged the threads in the holes.

RAY: After he made the mistake, the kid probably tried to "re-tap" the damaged threads in the cylinder head. A "tap" is like a sharp, heavy-duty screw that goes in and
"straightens out" damaged threads. But if he cross-threaded the TAP -- which is possible -- then he could have destroyed most of the threads altogether. And that's
probably what happened.

TOM: If I were you, I'd have your mechanic back out the remaining plugs and see what kind of condition the threads are in. If they've been damaged, too, he may have
to drill out more of the holes and put in inserts. But by doing that now, if necessary, you'll at least save you wife from thinking she's being fired on by NATO forces
when the next plug finally blows out.

Help safeguard your investment by ordering Tom and Ray?-s pamphlet Ten Ways You May Be Ruining Your Car Without Even Knowing It! Send $3 and a stamped (55
cents), self-addressed, No. 10 envelope to Ruin, PO Box 6420, Riverton, NJ 08077-6420.

?(C) 1999 by Tom and Ray Magliozzi and Doug Berman
Distributed by King Features Syndicate, Inc.

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