[Supras] Question about head gasket (hopefully last)

[berniek at technicaldevelop.com]

Mike:

    First, please bear in mind that I'm not trying to start a flame war.
I'll remain just with the facts, nothing more, nothing less.

    There is no denying that aluminum and iron have different thermal
expansion coefficients.  This is NOT simply theory.  It is fact, as can be
seen from the way a bimetal thermostat works.

    Stock head bolt torque appears to allow the head to "walk"
longitudinally on the block as a function of thermal cycling.  This was
evidenced by the recesses in the head and block I tore down.  Also, it may
be exacerbated by creep in the folded over and crimped sheet metal bore
inserts in the stock head gasket.

    With a metal gasket and sufficient bolt torque, the head and block will
be forced to remain together, although at temperature, the top of the block
will be in tension, and the head in compression.  That is due to thermal
expansion coefficients as follows:

Iron:  12 millionths of an inch per inch per degree centigrade
Aluminum:  24 millionths of an inch per inch per degree centigrade.

    However, metals can be constrained so as not to move, or to move
together.  This will be the case with a stiff (metal) head gasket and
clamping force attainable with studs.  This is not without its pitfalls, and
often results in head warping, since the section depth of the head is much
less than that of the block.  Metals can be compressed or stretched below
the point of permanent deformation according to the elastic modulus, the
ratio of applied stress to the strain (force) in PSI:

Iron:  30 million PSI per PSI
Aluminum:  10.5 million PSI per PSI

    Since the values for aluminum are lower than for iron, the head will
compress more than the block will expand, FOR THE SAME CROSS SECTION.
However, since the head is more shallow in section than the block,
practially all of the length change with respect for temperature will be
taken up in the head.

    When any solid, including metal is compressed, it changes in volume
according to yet another relationship, Poisson's ratio.  I'll respond about
this personally, not to the list, if anyone is interested.  Please do not
write to the list about Poisson's ratio.  Write to me and I'll respond.  On
a practical basis, it does not enter into head-block thermally induced
considerations.

        BernieK


----- Original Message ----- 
From: "theOnly_way2FLY" <supgrps1 at 57mgte.com>
To: "'t72pwrd'" <nholden1 at woh.rr.com>; <berniek at technicaldevelop.com>
Cc: <Supras at supras.com>
Sent: Sunday, June 04, 2006 10:17 PM
Subject: RE: [Supras] Question about head gasket (hopefully last)


If the cause of the blown head gasket on the 7M-GTE was because of
temperature differentials, poorly designed oil/water passageways, etc,
implying the head disfigures from the temperature changes, then installing a
MLS gasket would have no effect on this problem and it would still blow.

Yet, when a MLS gasket is *PROPERLY* installed in a 7M-GTE engine, it will
not blow, regardless of the levels of boost (assuming the other engine parts
can handle it).  A properly installed factory gasket into a 7M-GTE will
still blow from boost, eventually.


Y'all can theorize all you want.  I've built and re-built TOO many of these
engines to not have clearly seen the cause of the gasket failure - including
my own engine of 3 rebuilds.

Just my 2¢



Mike Lewitz
Phoenix, AZ
Supra - the only way to FLY!
==========================

Do you STILL believe 9/11 was a terrorist attack?
http://video.google.com/videoplay?docid=-8260059923762628848