Head gasketts.

Hi Roughasguts,
When an engine over heats, damage is often done to the head studs causing uneven tension of the head. The head also warps in most cases, once again, uneven tension on the gasket. On modern day engines head studs should be replace when head gaskets are changed.
Cheers,

A cylinder head in theory made up of different material thickness's that have a different thermal expansion rate due to the structural design.

A cylinder head can only be clamped in X amount of spots due to the position of water, oil, inlet and outlet ports in the head and block.

If an engine temperature increased at an even rate over the whole surface area' the head would not warp, in an engine you will always develop hot spots

When an engine over heats to a point where it will cause damage; your not talking about 110 Deg C or 130 Deg C you talking about temperatures much greater.

With modern computer design and thermal analysing they can develop a cylinder head with a much more even thermal loading capacity.

If the cylinder head expands or warps at a greater rate in one spot or the other the cylinder head will loose it's clamper force and the gasket will fail.

In some engines they do not have a head gasket, just two micro finished surfaces and in some the use o'rings.

As for a bolt stretching...for a bolt to change its structure and expand to a none acceptable leave where it fails, the amount of heat that would have to be generate for this to happen there would be a molten puddle of alloy, plastic and other low melting point materials... we are talking extreme heat of over 500 deg C.

Most head bolts on modern engine have to be replaced once the have been undone, these bolts are called "torque to yield" and are designed to stretch to a once only specification...... it has nothing to do with heat.

Gaskets are used for one reason and one reason only...... it cheaper and quicker in the manufacturing process of the components..... they don't have to worry about micro finished surfaces that are within .00005 mm's tolerance.

G'day all,
the headgasket usually causes the overheating! (faullty, old, loose) hoo roo

When an engine overheats the head studs do get longer and theoretically the head is loose on the gasket but the head expand too. While the head may warp at some point, the head, especailly aluminium heads expand at a greater rate than the head studs and try to clamp the gasket tighter than ever before, which reduces it's thickness and damages the gasket if it hasn't failed because of the additional heat.

Usually the gasket doesn't fail while the overheat is happening, it fails as soon as it cools down a bit and the now loose gasket and head are now held in place with head studs still the same tension setting. The gasket now thinner and head possibly warped, provides plenty of opportunity for the combustion gasses to leak outside, into water jacket, blow the gasket out of shape, develop cracks in head etc etc.

Most overheated engines continue to run as they overheat and get to the seize point sometimes as the engine tolerances are taken up by expansion, usually piston ring gaps first.
Remember, you DO NOT need oil or water in an engine to make it run they just make it last a bit longer. 100's of 1000' if possible.

Roughasguts - Head gaskets fail when an engine overheats, because the head expands substantially more than what the engineers designed for normal temperature ranges - and this crushes the gasket.

Once the gasket is crushed, it almost always fractures under compression loads, and then you have a "blown" gasket.
OEM gaskets are the cheapest construction the manufacturer can afford, and designed to be the weak point.
You can buy aftermarket gaskets that are superior, that have stronger fire rings around the cylinders, and which will have more "give" than the original gasket. These gaskets are able to withstand greater temperature variations.

However, an overheated engine often suffers other major damage, such as a warped head (common), a warped block (less common), warped valves, head or block cracks (usually between cylinders).
Except for warped valves, all of the aforementioned problems will contribute towards a head gasket failure as well.

It is important to examine an overheated engine carefully for all the aforementioned problems. Just installing a new gasket without checking the engine component condition and specifications thoroughly, will merely result in another head gasket failure within a short time after installation of a new gasket.

Cheers - Ron.

food for thought ...... i have picked up a ea falcon that a copper owned, he was driving at night and it overheated, blown hose, he just kept driving until it stopped, it got so hot it blistered the paint on the bonnet ......
So think about how hot it would be under the bonnet on a cold night to blister the paint on the bonnet, air flow over the motor, good 4 inches away from the motor ...... man that is some HEAT .... metal itself is warped in that heat so how do you reckon a head gasket goes given it is located at the exact point the heat is created ....
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