Just a minor point, perhaps, but I think worth noting: the normal reason for
a head gasket to fail on a bimetal engine is indeed differential expansion.
However, the mechanism of failure is more likely local relative sidewards
motions, since the head is held down very tightly, and the block is quite
stiff. The head is trying (locally) to slide across the gasket, or dragging
the gasket with it and trying to slide it across the block's face. Newer
construction gaskets often have a teflon or other material intended to let
that happen without harming the sealing ability. This may be explicitly
mentioned on the package if you buy one. Older ones, and some cheaper ones,
let the gasket try to deform to handle the expansion; eventually, a failure
occurs. It may be due to what is called "fretting corrosion", basically
chemical attack combined with small relative motions, or gross material
structural failure.
Now a second mechanism comes into play, as the engine overheats,
typically with hot combustion gases being delivered directly into coolant
passages and elsewhere. This produces drastic changes in local heating,
enough to distort the head, since combustion tempratures are well above heat
deflection [think "softening"] temperatures for aluminum alloys. When there
was coolant in the passages, the heat was carried away; now there is
combustion-heated gas warming the material inside and out.
It is also worth noting that a head - or a block, for that matter - may
have locked-in stresses from casting, and will relax and respond to themn
when heated sufficiently, or simply with the passage of time. A block and
head that were working fine, but which were separated for, say, valve work,
installing rings, etc., may show warpage when inspected, though this is
likely to be within tolerances.
There have been iron-heads and aluminum blocks; they are not immune to
the same kinds of gasket failure, though warpage is USUALLY a lesser
problem. Design details probably matter far more than the basic choice of
materials in such cases.
Valve cover gasket failure may also occur due to differential expansion,
and local sliding or twisting between parts. Thick gaskets help by
permitting some local deformation; over-tightening hold-downs defeats this
and bends the weaker component besides. Our 16-V cars don't technically
HAVE valve covers, and the manifold is aluminum like the head, but subjected
to far less heating. Such temperature differences can certainly do the job
of damaging gaskets, but they usually handle the motion much the same way as
valve-cover gaskets.
Ron
----- Original Message -----
From: "Brian" <[email protected]>
To: <[email protected]>
Sent: Monday, February 10, 2003 11:45 PM
Subject: head warpage questions(Re: [a2-16v-list] RE: consuming coolant)
> On Mon, Feb 10, 2003 at 08:07:15PM -0500, [email protected] wrote:
> > Aluminum heads are especially vulnerable to warpage and cracking because
> > aluminum has a much higher coefficient of thermal expansion than cast
iron.
> > Consequently, when a bimetal engine with an aluminum head gets too hot,
the
> > head tends to swell up in the middle, causing it to warp and blow the
head
> > gasket.
>
> So how do you tell if your head is warped? I guess you pull it off and
> then take it to a machine shop? If it is warped, can they grind it flat
> again and how much would such a thing cost?
>
> Also, I'm guessing the top of the block never has problems? ie, all you
> have to do is have the head fixed.
>
> I'm wondering if my oil leak that I think is the valve cover might also
> be coming from the head gasket leaking.
>
> If the head is warped, will that make the valve cover gasket leak, or
> will the valve cover warp with it? (Is the valve cover aluminum?)
>
> Laters,
> Brian
> --
> 1989 GLI Wolfsburg Ed.
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