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The cylinder head is located on top of the cylinder block. The bottom of the cylinder head is indented and combines with the piston to form the combustion chamber. Internally, there are a oil hole and a water jacket to cool the valves and the spark plugs. Most gasoline engines have cylinder heads made of aluminum alloy. Aluminum alloy is lighter than cast iron, and it has excellent heat conduction properties. In between the cylinder block and the cylinder head is the cylinder head gasket, which acts to seal the junction of the two parts in order to prevent leakage of highly pressurized gasses, combustion gasses, coolant and engine oil.