newbie question - please don't laugh...

Right, first. A gasket is a seal which fit between two components in order 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 from "Owen" 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...

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

Thanks Guy, heplful info and useful pics too!

Explains a lot !

Owen

overheating -

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 from "Owen" contains these words:

Quite possibly. Is the murky stuff oily as well?

overheating -

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 saying something like:

Vaguely Useless Info Dept:

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.

Grimly Curmudgeon ( snipped-for-privacy@hotmail.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.

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Adrian saying something like:

I think it was the way to do things for reliability, at least as far as head/block sealing was concerned. I suppose if head gasket sealing wasn't all that good (certainly RR found that out) the best way of doing away with the problem was avoid it entirely. I'm sure that cheaper marques used a separate cylinder head from the early days, but the richer buyers were buying reliability and didn't mind paying for it.

The message from Adrian contains these words:

This bunch still use 'em...

It seals between the cylinder head and cylinder block.

Usually they are brown or grey but when they fail become blue ;-p