Need to use special expansion, assembled valve is used as as a positioning benchmark, expanding both ends. Forged steel gate valve body recover more than the amount of spring seat rebound, the valve body hole tightly surrounds the valve seat, does not have the gap, the seal is good. Therefore, it is necessary to strictly control the axial load. When the valve seat pressed to the valve body, the inner wall need be deformed within the elastic limit, to ensure that the expansion force disappears, the valve body wall rebound to make up the valve seat rebound, it is necessary to limit the maximum axial load.