In an internal combustion engine, a head gasket provides the seal between the engine block and cylinder head(s). Its purpose is to seal the combustion gases within the cylinders and to avoid coolant or engine oil leaking into the cylinders. Leaks in the head gasket can cause poor engine running and/or overheating. Purpose Within a water-cooled internal combustion engine, there are three fluids which travel between the engine block and the cylinder head: Combustion gases (unburned air/fuel mixture and exhaust gases) in each cylinder Water-based coolant in the coolant passages Lubricating oil in the oil galleries Correct operation of the engine requires that each of these circuits do not leak or lose pressure at the junction of the engine block and the cylinder head. The head gasket is the seal that prevents these leaks and pressure losses. Types Multi-Layer Steel (MLS): Most modern engines are produced with MLS gaskets. These consist of two to five (typically three) thin layers of steel, interleaved with elastomer. The contact faces are usually coated with a rubber-like coating such as Viton which adheres to the engine block and cylinder head while the inner layers are optimized for resilience. Solid copper: A solid sheet of copper, and typically requires special machining called O-ringing that places a piece of wire around the circumference of the cylinder to bite into the copper. When this is performed copper gaskets are very durable. Composite: An older design which is more prone to blowouts than newer designs. Composite gaskets are traditionally made from asbestos or graphite. Asbestos gaskets are increasingly rare due to health concerns. Elastomeric: Uses a steel core plate with molded in place silicone rubber beads to seal oil and coolant passages. The bores are sealed by rolled steel fire rings in a more conventional manner. This type of gasket was used in the Rover K-series engine. O-ring: These gaskets are typically built from steel or copper. They are reusable and if used between correctly prepared flat surfaces will yield the highest clamping pressure, due to their much lower surface area compared with other gasket types. Gasket failure A leak in the head gasket - often called a "blown head gasket" - can result in a leak of coolant, the combustion gasses, or both. Blue smoke from the exhaust suggests that excess oil is entering the combustion chambers (although there are other possible causes than a head gasket