[Supras] Question about head gasket (hopefully last)

[Hacker J]

Bernie,

I forgot to mention that the MLS gasket is designed to allow some slip as the engine warms up.
There are some pictures floating around on Supramania that show polished areas on a head from a
cometic gasket sliding back and forth (IJ's build thread I think).

There is an informative article on MLS headgaskets here...
http://www.babcox.com/editorial/us/us80222.htm

Jon
90T

> > > From: berniek at technicaldevelop.com
> > Subject: Re: [Supras] Question about head gasket (hopefully last)
> > CC: Supras at supras.com
> > Date: Mon, 05 Jun 2006 00:05:40 -0400
> > To: theOnly_way2FLY <supgrps1 at 57mgte.com>, 't72pwrd' <nholden1 at woh.rr.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
> > 




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