Head gaskets seal and contain the pressures of combustion within the engine, between the cylinder head and the block.
Some high temperature head gaskets are called 'anisotropic' in nature. This means that the gasket is designed to conduct heat laterally to transfer heat from the engine to the coolant faster. They are normally constructed with a steel core. Special facing materials are added to both sides of the gasket core to provide a comprehensive seal under varying torque conditions.
Some head gaskets incorporate stainless steel fire rings to help to contain heat and pressure within the cylinder. Many head gaskets also have an added silicone based outer coating on both sides of the side material layers to provide additional cold sealing ability during start-up and warm-up. Head gaskets also seal oil passages, and control the flow of coolant between the cylinder block and head and are fitted with beads or rings to prevent leakage and corrosion.
Gasket manufacturers have produced improved material combinations such as nitrile and cork blends because pure rubber, or conventional cork-rubber, is unable to deal with the stresses and pressures of 'high tech' engines. Such combinations are more able to deal with issues such as compressibility and wicking.
Some materials are designed to 'swell' in application and increase sealing ability. For instance when oil inside a valve cover penetrates the edge of the gasket material, it is designed to swell by approximately 30%. This swelling effect increases the sealing pressure between the head and valve cover sealing surfaces and helps to seal potential leaks.
Some materials are also designed to allow the cylinder head and block, some of which have considerable distortion rates, to move slightly on the head gasket as they expand during engine warm-up. This feature can be vital for preventing head gasket failure.