Re: untightened sparkplugs

["Smeins, Larry" <lsmeins@ball.com>]

There are more important considerations than leakage related to loose
sparkplugs.  The plugs depend on good thermal contact to the head for
temperature control.  A loose plug will run too hot with all the
resulting effects.  The worst of these is pre-ignition and melted
pistons.

Some years back I had one of a new set of plugs back out of my 5kcst.  I
first noticed the problem by a miss under acceleration on my way to work
one morning.  I had twenty miles to go and continued on to work with
restraint on the throttle.  By the time I got to work I was running on
4.  I went out at lunch and started checking.  I found the loose plug
and tightened it with no fix of the miss.  When I pulled the plug I
found there was no center post in it.  I was sure the plug had failed
and had dropped its innards into the cylinder.  I borrowed a bore scope
and went looking for the pieces, expecting them to be embedded in the
piston top.  I found nothing but a cylinder and piston in good shape.
Replaced the plug and the engine ran fine.  After much  thought I came
to the realization that the plug had gotten so hot it literally melted
the center electrode away and the early miss under acceleration was a
precursor as the electrode was being eaten back.  I consider myself
lucky that I did no more damage.

I have been putting plugs into aluminum heads for many years and felt I
had developed a good feel for the proper torque but this one time I
failed.  I was surprised that Bentley had no torque spec for the plugs.

The story has another chapter relating to the nasty letter I wrote to
the president of NGK America before realizing what actually happened but
I won't waste the space telling it.

Larry



>Date: Sat, 6 Jun 1998 22:47:23 EDT
>From: C1J1Miller@aol.com
>Subject: Re:  untightened sparkplugs

>In a message dated 6/6/98 4:21:43 PM, pjrose@servtech.com wrote:

>
>Is it possible that _un_torqued spark plugs will _not_ degrade engine
>performance? I wonder if this will depend greatly on rpm and engine
>temperature?

>If the plugs were fully threaded in, then I wouldn't expect the minimal
blowby
>to affect performance.  After all, the threads don't leave much room
for
>escaping gases.  If they were partially unthreaded, you've increased
the
>height above the piston, and could have an inefficient or partial burn
>pattern.
>Chris