[s-cars] AAN Sparkplugs from 94 Quattro

[Jeff Gerner]

Hello guys – this is my first EVER post on this site so I am “breaking my
cherry” in order to shed some light on this sparkplug debate. I am a
lifelong auto tech and engine builder as well as the owner of a very fast 94
S4 that I have “done all the work on” etc. I am no kid either at 54 years.

 

Anyway, on to the subject; Rule # 1. Properly tightened – notice I did not
say torqued – spark plugs, just like properly tightened wheel lug nuts /
studs, don’t come out. Period!

 

 The spark plug loosening problem happens for two reasons; First is the
highly dissimilar “growth rate” of aluminum vs. steel / iron as in the
cylinder head spark plug interface – the aluminum cyl head grows or swells
more in dimension (under extreme heat) than does the spark plug shell. This
causes “cyclic working” of this joint between plug and head as the engine
cycles repeatedly from cold to very hot.  This problem is exacerbated by the
rather extreme cylinder pressures and temperatures associated with high
boost engines.

 

 The second and most causation related factor is that the factory listed
torque value for the spark plugs is insufficient. The reason it is
insufficient is the relative disparity in the “crush load” requirement of
the spark plug sealing rings. In other words the factory specified spark
plug torque rating is insufficient to uniformly and consistently crush the
little soft metal rings that seal the spark plug. (Remember the cyclic
working of this joint)

 

Here is what happens; Stock torque spec fails to sufficiently compress the
seal ring leaving more room for further compression of the seal ring during
heat cycling of the joint between head and plug which ends up leaving the
plug loose when it is cold!  This will eventually let the plug loosen enough
to work its self completely out over time.

 

The fix; When installing new plugs VERY lightly oil the top half of the
threads and the seal ring to aid the seal in crushing and then tighten the
plug until you feel the seal compress and the plug become fully tightened.
You will feel the initial resistance of the seal meeting the head surface
and you will feel the crushing of the seal ring as you slowly increase the
torque. You will know when the plug is “properly tight” when the resistance
to tightening suddenly increases. If you have any “feel” for these type of
things you will not have any trouble doing this. If you don’t trust yourself
just have someone who is a full time wrench do the job.

 

Note; some people will screen “blasphemy – must use torque wrench when
installing plugs”  The truth is that only 1 in 50 experienced techs use a
torque wrench to install spark plugs – they don’t need it because they can
feel when the plug is tight enough to stay in while still not being over
tight.

 

Hope this helps. 


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