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What is a Head Gasket?

By Lori Kilchermann
Updated Jan 31, 2024
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A head gasket is used to seal the combustion chamber by filling the gap between the engine block and the cylinder head. Consisting of composites surrounding a thin metal layer, the head gasket is flattened to size when the cylinder head is torqued into place. Typically a one-time use type of component, some special solid copper or aluminum head gaskets crafted from material in dead soft hardness are capable of multi-use applications. Used without any further sealing compound, this type of high-performance head gasket is capable of effectively sealing the combustion chamber as well as the water passages against any leaks.

Some head gasket types utilize a graphite compound on the outer surface of the gasket. This graphite material acts as a sealant and ensures no leaks develop over time. As the engine comes up to normal operating temperature, the graphite bonds with the head and block surfaces to create a permanent bond and seal. When disassembled, this graphite is often very difficult to remove and typically requires a razor blade or a sturdy putty knife to scrape it clean. Most engine manufacturers recommend a small dab of silicone sealant be applied to the corners of the head gasket as well as around the coolant ports to provide an extra measure of protection against leakage.

One of the most important factors in creating a proper seal between the engine components and the head gasket lies in the proper machining of the cylinder head and engine block surfaces. Creating a smooth and level surface is not enough when machining the pieces. Providing the correct surface to dig into the gasket is a matter of great importance. If the surface is too smooth, the gasket will not have anything to bite into; if it's too rough, the gasket will not be able to fill the crevices. The machinist must know which type of head gasket will be used to ensure he puts the proper surface on the parts as they are machined.

When combining contrasting materials, such as using aluminum heads on a cast iron block, special head gaskets must be used to effectively seal the engine. This is due to the different expansion rates of the two different metals. Failure to use the proper gasket will result in the gasket being torn apart, as the two sides are stretched at different rates as the engine heats up. Many head gasket issues come from the failure to use the proper type of gasket.

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Discussion Comments

By anon319095 — On Feb 11, 2013

@Kentuckycat: A blown head gasket means that you're engine is leaking combustion pressure (cylinder pressure), i.e., part of the pressure created by the ignition of fuel and air with spark seeps out. It can also leak coolant. It is in between the head and the block, it is not in the cylinder, nor around the piston. Basically, the block houses the crankshaft, conrods, pistons, cylinders which are usually pressed into the block under massive hydraulic pressure. The head gasket sits on top of the block and is responsible for holding the combustion pressure in.

They are expensive as they are massively time consuming. Also, as a rule of thumb, if the head is off, you get it checked at the bare minimum. I just did mine and it needed reconditioning. That alone would have cost me about $700 AUD if I weren't in the industry. Then you should really machine the block, which requires taking the whole engine out, dismantling the block, cleaning and inspecting it, machining the mating surface then re-assembling. A typical head gasket will vary but can be between $50-$250-plus depending on composition, e.g., metal,cork graphite, etc.

Diesels have head gaskets, yes. Usually, they all run metal or composite layer gaskets because the cylinder pressure is a lot higher in a diesel.

By anon319092 — On Feb 11, 2013

@Jimmy t: Products like chemiweld, silver solder, barrs, etc., are all temporary fixes. Essentially, they are designed to only activate when they come into contact with air. Most are very good at blocking water jackets and radiators over time, though unfortunately which is why they are usually temporary. I have heard of guys getting huge kilometers/miles out of chemiweld, I wouldn't recommend it. It is also a nightmare to clean water jackets and radiators afterwards.

By JimmyT — On Jan 15, 2012

Clearly, all internal combustion engines have head gaskets around the cylinders, but what about diesel and other types of engines? Since diesel engines have cylinders, I would assume they also have head gaskets, right? Are there any different blown head gasket symptoms for a diesel engine as compared to a gasoline engine? I know the two types work a little bit differently.

I have also heard about products you can buy that are supposed to repair a head gasket without taking the engine apart. Do these things really work? I'm not a mechanic, but from what I know about how the head gasket fits in with the engine and what happens when it breaks, it seems like it would be impossible to just put some sort of special liquid in your gas or oil or somewhere and have that seal off the leak. Has anyone ever tried this, and does it work?

By Izzy78 — On Jan 14, 2012

@kentuckycat - I would add, too, that there are several effects a head gasket problem can have. I think the most common one was mentioned, but sometimes you will have a head gasket problem, and it won't really be noticeable until all the coolant leaks out and the engine starts to overheat. In general, if you ever notice the engine start to overheat, pull over and get the car towed to a mechanic, because all you can do is cause more damage at that point.

I know I had to get a head gasket replacement one time, and it cost in the neighborhood of $1200. I'm not sure exactly what causes the problem initially, but from what I've been told, they just sort of wear out after a while. Apparently the metal expands and contracts, and it can just give out and crack. That being the case, I would guess it's easier to have head gasket problems in colder climates than warm, but I don't know that for a fact.

By stl156 — On Jan 13, 2012

@kentuckycat - It's always good to be able to recognize when you might have a problem with the head gasket, because head gasket repair is expensive to start with, and not realizing it could potentially destroy your engine.

I was riding with a friend once when he had a head gasket go out. Basically, it was cold at the time, and we first noticed that the heat in the car stopped working. Then the temperature gauge shot up, and we could just tell that the car wasn't running like it was supposed to. We eventually had to pull over and get the car towed away, and learned later what the problem was, since neither of us had experienced anything like that before.

I remember reading about it at the time, but I don't remember exactly what happens when it goes out. Like the article says, I just remember that somehow the gasket interacts with the engine coolant, and stops it from getting inside the cylinder, and that was what happened.

By kentuckycat — On Jan 13, 2012

So, I hear a lot of people always talking about a "blown head gasket," but what exactly does that mean? Is the head gasket actually the seal that goes around the piston while it's inside the cylinder, or is it outside of that? I'm not sure I completely understand how it fits in with the rest of the engine.

Also, why are blown head gaskets to expensive to get fixed? Is the part itself expensive, or does it just take a lot of work to get to it and repair it? How exactly do you know that you have a problem with the head gasket, and what should you do if you notice it? I'm just curious in case it would ever happen to me. I'd be interested to know what causes it, as well.

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