Gaskets and seals the secure machined joints, and prevent fluids and gasses (oil, gasoline, coolant, fuel vapor, exhaust, etc.) from leaking. The cylinder head has to keep the water in the cooling system at the same time as it contains the combustion pressure. The head gasket is one of the most important parts in an engine because it has to seal all of the combustion chambers as well as the coolant and oil channels between the head and block. It has to provide a leak-free seal from the moment it was installed and uphold that seal for the life of the engine – which might well be 150, 00 miles or more on a lot of today’s automobiles.
One reason which makes many engines hard to seal these days is aluminum cylinder heads on cast iron blocks. Aluminum heads has less weight but when they get hot, they expand 1.7 times faster than cast iron. That makes it tougher for the head gasket to uphold a seal with all that thermal cycling with iron heads. In other words, aluminum heads cannot withstand overheating as iron heads could in the past. Another is that, if the head gasket can’t bear up this motion and if it’s not strong enough to withstand the shearing forces that occur every time the engine is started, run and allowed to cool back down, it will leak eventually. Gaskets made of steel, copper and asbestos are used between the cylinder head and engine block. Because the engine expands and contracts with heating and cooling, it is easy for joints to leak, so the gaskets have to be soft and "springy" enough to adapt to expansion and contraction. They also have to make up for any irregularities in the connecting parts.
Head Gaskets are a costly part to replace.