[Supras] Question about head gasket (hopefully last)

[Hacker J]

Bernie, it is interesting to look at some of the differences in the 2JZ and 7M engines. Notable
points are that the head gasket is metal, as mentioned before, but also the 2JZ water cutouts are
much smaller, making the whole gasket much more rigid.  You have probably seen deformed water
cutouts in the stock gasket if you have experienced a BHG. Also of merit is the cylinder pitch. 
The 2JZ has larger lands between cylinders. That gives the 2JZ gasket more sealing area than the
7M. The M series started out with much smaller 75mm bores, and gradually worked its way up to 80mm
(4M) and finally 83mm (5M, 6M, 7M). So the lands between cylinders has shrunk over the years while
the power went up. The 5M had essentially the same gasket (and bolt torque) as the 7M, but did not
suffer from BHGs like the 7M. Presumably the lower power (145hp) helped, along with a simpler and
better cooled head design.  The 5M did have worse cam wear though. The 7M does have the thermostat
moved to the exhaust side to help cool things down a bit compared to the 5M, but evidently not
enough. 

The link shows the 7M and 2JZ head gaskets side by side (courtesy of Aaron).
http://img.photobucket.com/albums/v379/BigAaron/misc/DSCF2589.jpg  



> > 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
> 
> 
> ----- 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!
> ==========================


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