What is a Head Gasket?
The critical seal between the engine block and cylinder head that keeps coolant, oil, and combustion gases apart.
Simplified animation — not to scale.
In plain English
The head gasket is a thin, multi-layered steel or composite seal that sits between the engine block and the cylinder head. Its job is to maintain pressure inside the combustion chambers while keeping engine oil, coolant, and exhaust gases from mixing together.
"Like the lid on a pressure cooker — it has to keep steam in, stop liquid escaping, and withstand intense heat, all at the same time."
How it works
Bolts clamp the cylinder head down onto the block with enormous force, squeezing the head gasket so it fills every tiny imperfection between the two surfaces. It seals the tops of the cylinders for compression, channels coolant around the combustion chambers, and routes oil to the camshaft and valves — all without letting any of the three fluids meet.
Signs of trouble
- ⚠White smoke from the exhaust that smells sweet
- ⚠Engine overheating or running hot
- ⚠Milky, mayonnaise-like substance on the oil filler cap
- ⚠Coolant disappearing with no visible leaks
- ⚠Loss of power and rough running
- ⚠Bubbles in the coolant expansion tank
£450 – £1,500+ (often the most expensive engine repair)
Always get a written quote. Prices vary by car, region, and parts brand.