What's a head gasket?

In fact, what's a gasket?

And how can it 'fail' ?


Cheers
Owen

"Owen" <spam (AT) spam (DOT) com> wrote

to keep various fluids out or in as necessary. These vary from a simple
piece of paper with holes in it to a complex "sandwich" of metals, asbestos,
or whatever the manufacturer considers appropriate. A head gasket, or
cylinder head gasket to give it its full title, is a gasket fitted between
the cylinder block and the cylinder head of your car engine. Look at the
engine just below the spark plugs, and you'll see the join. This is where
the head gasket lives. The head gasket has loads of holes in it of varying
sizes, some of which have oil passing through them, some have coolant, some
have the securing bolts (head bolts). And there are 4 (or more, according to
the No of cylinders) large holes for the cylinders/combustion chambers.
These large holes are subject to the very high temperatures and pressures at
the heart of your engine.

Now, how they fail. They can fail in a number of ways, all bad. With a
simple paper gasket it's not too bad, but with a head gasket, failure means
that one or more of the holes has become connected to another one which is
subject to a different fluid and the pressures involved with it, allowing
oil and water to mix, or combustion gases to enter the cooling system. This
latter is generally regarded as the worst case scenario.

The only cure is replacement with a new gasket, which means removing the
cylinder head, which in turn means according to the engine you've got that
various other things have to be disturbed and then require replacement
themselves. Many engines nowadays for instance, have head bolts which are
only used once. It is also good practice to replace the camshaft belt whilst
it's off, especially if you can't be certain of its age.

Steve

The message <nRPZe.20072$ws4.10210 (AT) newsfe5-win (DOT) ntli.net>
from "Owen" <spam (AT) spam (DOT) com> contains these words:

Engines can't practicably be made in one piece. The bit that the pistons
go up and down in is made seperately from the bit the valves run in and
through which the inlet and exhaust tracts run. They are bolted together
with lots of very tiht bolts to contain the pressures created by the
burning of the fuel and also the cooling water etc. However, life being
what it is, the machined surfaces however good will never be totally
perfect, so some sort of seal is required between the two bits (called
the block and the head).

This seal is a head gasket. They're between 2mm and 4mm thick, with
metal rings to protect the exposed edges of the gasket material where
the holes for the pistons are.

Something that creates a seal between two mating faces.


Often the top bit of the engine - the head - is a different material
from the block. This means that they expand and contract at slightly
different rates. This puts strain on the gasket. Also the combustion
gases and hot and corrosive which may eventually break through the metal
ring sealing that bit.

That and probably a thousand other things.

The result of the failure depends on which bit of the gasket has failed.
There are three things to keep apart and in. Oil, water and combustion.
If the gasket has failed between water and oil, you'll get water in the
oil or oil in the water or both. If it's failed bewtween the bores and
the water you'll get pressurised cooling water system and sometimes
water in the bores (pistons can't come up 'cos the water's in the way).
If it's failed between oil and the outside world you'll leak oil. And so
on.

Replacement involves considerable work (hugely variable depending on the
engine and the car) to dismantle the engine. The head may then need to
be machined to make sure it's dead flat. Then there's all the faff of
putting it all back together again.

This is an engine with the top off and no gasket fitted...
http://www.virgeweb.com/rage2/944t/2...0Block%202.jpg

And this is the same engine with a new gasket on, waiting to be reassembled
http://www.virgeweb.com/rage2/944t/2...d%20Gasket.jpg

--
Skipweasel.
In the beginning was the word.
And the word was Aardvark.

"shazzbat" <shazzbat (AT) spamlessness (DOT) co.uk> wrote


Ah !

So if my coolant has gone a murky dark colour and my engine is overheating -
it could quite possibly be because my head gasket is broken and letting oil
and/or combustion gases into the cooling system, or other stuff into the
wrong place where it shouldn't be.

Cheers, starting to make sense now.

Owen

"Guy King" <guy.king (AT) zetnet (DOT) co.uk> wrote


<snip>

Thanks Guy, heplful info and useful pics too!

Explains a lot !

Owen

If it's a more grey/black colour then that's more likely, but don't discount
the fact that they could have mixed coolant types, which causes an unwanted
chemical reaction between the different types of antifreeze (which a lot of
mechanics may not know or understand) and somehow causes a loss of coolant
(maybe it seeps out through things more easily - all I know is that this is
what happened in my TDi Fiesta), which could lead to overheating.

What engine is it?

Peter

The message <lxQZe.19590$st1.17822 (AT) newsfe3-gui (DOT) ntli.net>
from "Owen" <spam (AT) spam (DOT) com> contains these words:

Quite possibly. Is the murky stuff oily as well?

--
Skipweasel.
In the beginning was the word.
And the word was Aardvark.

"Owen" <spam (AT) spam (DOT) com> wrote

I should have added, that your garage should have the fairly simple tester
which will tell them whether combustion gases are getting into the coolant.
It may be worth asking for this to be tested before allowing them to rip
your head off.


So to speak :-))

Steve

We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Guy King <guy.king (AT) zetnet (DOT) co.uk>
saying something like:

Vaguely Useless Info Dept:

<tale recollected from 30+ years ago from ex RR mechanic, so might be
wrong in detail>

MkI Spitfires were fitted with a Merlin which had a one-piece block and
head casting, ie, no seperate cyl heads. Obviously this had its
drawbacks when it came to valve replacement as it meant the whole engine
had to be taken out and stripped. RR designed a new engine with seperate
heads but the head gaskets weren't quite all that, with the resulting
problems associated. An interesting but sometimes fatal problem of
water/glycol being burned in the combustion chamber was a heavy vapour
being discharged from the exhausts. Not a problem in flight, but when it
came to landing some guys didn't make it because they couldn't see out.
--

Dave
SE6a

Grimly Curmudgeon (grimlycurmudgeon683REMOVE (AT) hotmail (DOT) com) gurgled happily,
sounding much like they were saying :

One-piece heads and blocks were quite common in "the early days" - Bugatti,
IIRC, used them quite a lot and were by no means alone.