Miata Mailing List: July 1996, Message #157
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   From: afn34478@afn.org (Robert Pace)
Subject: Miata head aka spark plug rocket launcher
   Date: Tue, 2 Jul 1996 14:03:42 -0400

The strangest thing happened to me. A couple of months back my engine
started ticking. The sound I recognized as exhaust gases escaping and my
conclusion was that I had burned through the exhaust manifold gasket.  It
was not bad and I was busy with work so I waited.  Then the ticking became
much worse and it was obvious that it was exhaust gases escaping.  I parked
the car and waited for the appropriate gaskets to arrive (thanks Roebuck)
and hoped that the exhaust manifold was not cracked.

Well, as I was about to start the repair, a friend, and more experienced
mechanic, noticed that a spark plug wire appeared to be loose...and not only
that but the spark plug as well.  I tightened the plug back down and put the
plug wire back on and the car ran like a champ again.  I thought it was
strange that it had taken nearly a year for the plug to back out from the
last time I changed plugs.  But, I figured it could happen.

Two days later while sitting at a light the car suddenly begins to run rough
again and before I can pull off the road....  And the car dies.  I
manage to get it to the side of the road on the starter, I pop the hood,
jump out and find my wiring harness burning where a spark plug was resting
after being launched from the cyl head.  I pull my shirt off and used it to
grab the plug and throw it to the ground.  Luckily, I was idling so the plug
did not damage the hood, if I had been at 7000rpm I'm sure my hood would
have looked like it had been hit by a 20mm cannon.  But I figured my cyl
head was going to have to come off to fix what was undoubtedly stripped threads.

Once the head was off it was obvious that was not the problem.  The threads
were not in the best shape but they were not stripped.  After fooling with
it and getting a friend who has more experience with aluminum heads to look
at it, we found that the plugs were loose.  With the plug screwed in hand
tight the plug had a significant wobble.  It was the same on three of the
four plug holes. Bad plugs?  Nope, they were Autolite Double Platinums but a
set of brand new NGK's were just as loose, and according to a micrometer
both brands were within .001 of each other in diameter.

Now with heli-coils in place the wobble is gone...but I still don't
understand how this could happen.  I have an aluminum cyl head phobia so I
am always careful when installing plugs and I use anti-sieze compound as
well.  Anybody got an idea why this happened.


Robert Pace
TR-4A (no bloody IRS)
Miata (aka TR-9)
Have fun, drive fast, drive safe, don't get scared. <:o
"Whatsa behind me is nota important" Gumball Rallye