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.
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