"iBoaterer" wrote in message
...

In article 1444394091378758162.549500bmckeenospam-
, says...

iBoaterer wrote:
In article ,
says...

On 12/31/2012 5:44 PM, Wayne.B wrote:
On Mon, 31 Dec 2012 15:23:44 -0600, Califbill
wrote:

The fix is not stronger than the base metal, but there
is more base metal involved. Therefore the fix is stronger.

===

Sounds right to me. The heli-coil is bigger than the plug so it
has
more contact area (gripping surface) with the aluminum head.
All
other things being equal, it should be stronger than the
original.


What I have been saying all along..

You are wrong. Once again, the base metal THICKNESS has not
changed. the
base metal thickness is what gives it X in tensile strength (or
compressive strength for that matter, although here we are
strictly in
tension) Okay, now there is a cone of influence DIRECTLY
proportionate
to the thickness of the base metal. You only WEAKEN that base
metal by
enlarging the whole. The perfect cone of influence is a 45 degree
angle,
conical of course. IF that cone of influence doesn't fully develop
because of a lack of thickness of base metal, then it's weaker
than it
could be. I doubt you and your dummies will get it, but I'm sure
Wayne
will.

OK. Use this example. You use a 10-32 screw in a sheet of metal.
Does
not matter what material really. How much force to pull out that
screw is
required? Now, same piece of base material. Drill and tap for a
4"-32
screw. Install screw. How much force required to pull that screw
loose?
You should not make big bets without knowing the odds. Same
principal if
you welded on a pad-eye and increased the size of the pad. Takes
more
force to rip it loose.

You stupid old fool!!!! What *I* said was that the fix is NO STRONGER
THAN THE BASE METAL... But, I WILL take the bet. What you are failing
to
understand, or know, or whatever, is the CONE OF INFLUENCE. Do some
reading. And yes, it certainly DOES matter what the material is. Do
you
really think that the above screw would have the same tensile pull
resistance in pot metal or case hardened A325 steel??? I guess then it
would have the same resistance in plastic, too?

Now, to your above example. What you are failing to grasp is that the
base metal has a given shear resistance strength. That strength is
DEPENDANT on the thickness of the threads. We know the threads per
inch
count is the same, therefore the tensile resistance is the same. You
are
confusing the bolt properties with the base material properties.

----------------------------------------

The design of the aluminum heads used where particularly thin in the
spark plug area. The spark plugs in question had a 3/4" long
threaded section, yet only 4 threads engaged in the head when properly
torqued. That, plus the use of an alloy that weakened with
repetitive heat cycles are the reason for the material failure. I am
not a mechanical or materials stress engineer, but I don't think the
minor extra "diameter" of a heli-coil makes any difference at all.
The problem is the thin heads with minimal threads in a weakened
alloy. A heli-coil doesn't add any additional threads.

The way I see it, the only "load" placed on the head by the spark
plug is the resultant torque stress from installing. Other than
that, the spark plugs don't place any additional stress load on the
head. What *does* impart a stress load is the compression cycle of
the cylinder.

It's noteworthy that a majority of the failures occurred when the
engine was under a heavier than normal load ... hauling or towing.
This would result in higher RPM, more localized heat and a higher
level of compression cycles.

It's also noteworthy that trucks with the heli-coil "fix" also
experienced failures. Some lasted 6 months. Some never blew again.
I think it depended on how the truck was used.