The valve seat is pressed into the cylinder head and then machined for proper fit and tightness. The valve in the valve seat closes the part of the combustion chamber of the internal combustion engine facing the cylinder head. When renewing the cylinder head, if the valve seats are worn, they need to be replaced. The new seats are pressed into place using a special press. The valve seat is then milled using modern valve seat milling machines. The material of the valve seat is rarely made of the material of the cylinder head, of course in the case of a cast iron cylinder head. Valve seat materials: special Cr steel, alloy casting, sports, tuning case use beryllium material or powder metallurgical valve seat. Gas engines require Ni-based valve seat materials.