[s-cars] AAN Spark Plug Back-out

Vincent Frégeac s.sikss at gmail.com
Fri Nov 16 07:46:41 PST 2007


If I may jump in, the method in the madness of the double 22 lb-ft theory
has to do with the material elastic and plastic behaviour.

The short version, if you already know the elastic-plastic-break curve of
materials, is: The crush washer is a spring. At 22 lb-ft, you're only
halfway through the elastic zone, so it will spring back (elasticity) if you
back it out, the same as a suspension spring compressed halfway. A
suspension spring will spring back with no permanent deformation but the
crush washer is a poor spring design (compared to a coil that wouldn't fit
there anyway) made in cheap material (compared to a suspension coil
material). So even halfway through the elastic zone, there's already some
permanent deformation (plasticity), which is a good thing in this case. By
torquing once at 22lb-ft you deform all the crush washer to the same spec,
thus removing some of the original variation from the manufacturing process.
When you retorque, you have crush washers with have much less variation so
your sparkplug are more evenly torqued than the first time. Now, heat
cycling (driving) a compressed spring may deform it depending on the
material, and probably do with the cheap material used in crush washers,
even when within its elastic zone. So it's not advised to back out and
retorque a sparkplug after driving because you don't know how much
elasticity the crush washer have lost so you have no way to know how much it
should be torqued now.

You can check the "only halfway through the elastic zone" by comparing
22lb-ft torque to the torque by feel where you basically go through all the
elastic zone (slowly increasing torque) then a bit through the
plastic/deformation zone (rapidly increasing torque). 22lb-ft is well within
the slowly increasing torque, i.e. elastic zone.

Now, when you torque by feel, you're deforming the crush wahser, though not
killing it really, it still has some spring left. So it's not advised, in
that case, to back out and retorque, because, as after heat-cycling, you
have no way to tell how much you've killed the crush washer and how much you
should retorque it.

That was the "short" version. Now, the long version, with explanation of the
elastic-plastic-brake curve is on demand only because it's really long.

Vincent now cooking eggs on his forehead F.

-----Message d'origine-----
De : s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com] De la part de David Forgie
Envoyé : 15 novembre 2007 02:52
À : s-car-list
Cc : QSHIPQ at aol.com
Objet : [s-cars] AAN Spark Plug Back-out

Scott:  I am not following your logic ("method in your madness") of your

"I use the same procedure many seasoned audi shops use,  torque all to 
22lb/ft.  Then go back 1 by 1 and loosen and retorque to  22lb/ft." 

(Note: I think the "lb/ft" should be lb ft, like Newton metres)

To me, backing the out the plug, manually, on purpose, kills the crush
and the when you go back in and retorque them there is no more crush washer
to provide what I thought is the required "tension" on  the threads, 
i.e. the crush washer acts a spring, pulling the upper side of the plug
threads against the lower side of the head threads.

What am I missing?

I am not being arguementative, I just don't understand why my thinking is
the opposite of yours. 

Dave F. 

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