Euuuhh... Mechanic 101 :-)

Head gasket failure (any vehicle)mean that coolant can leak outside OR/AND
inside the combustion chamber.

Head gasket job: to seal combustion gas, oil passage and coolant passage to
the head.

Picture:
http://www.benplace.com/amc_head/amc9.jpg

The 2 inner flat ring seal the combustion gas from going anywhere else.
The big water jacket rubber seal is there to keep coolant from comming out
of the engine.

The little sleeve/cylinder outside green O-ring stop the coolant from
leaking into the combustion chamber.
http://www.benplace.com/bjp_head2/100_0043.jpg

And the little black O-ring (bottom outside sleeve/cylinder) is there to
make sure coolant doesn't end up in the oil... or oil in coolant.:
http://www.benplace.com/bjp_head2/100_0042.jpg

The pressure of the piston compression is way higher than the coolant
pressure, so this is why the first sign of a failing head gasket is
overheating. Coolant that was well bleed before become full or air bubble
and the coolant doesn't circulate properly.

So when one of those start to fail, the others will follow soon.

A strange design it is...
http://www.benplace.com/bjp_head2/100_0036.jpg

Ben

On Wed, 16 Dec 2009 05:29:51 -0500, Woody Halsey <WHalsey@SYA.ORG> wrote:

>Can anyone explain to me how a head gasket job would fix the problem, if
>there is no visible leak?
>
>Thanks.
>
>Woody
>83.5 V'gon
>Haverhill, MA
>