 For certain applications the valve stem and disk are made of different steel alloys, or the valve stem may be hollow and filled with sodium to improve heat transport and transfer.Although a better heat conductor, an aluminium cylinder head requires steel valve seat inserts, where a cast iron cylinder head would often have employed integral valve seats in the past.Because the valve stem extends into lubrication in the cam chamber, it must be sealed against blow-by to prevent cylinder gases from escaping into the crankcase, even though the stem to valve clearance is very small, typically 0.04-0.06 mm, so a rubber lip-type seal is used to ensure that excessive oil is not drawn in from the crankcase on the induction stroke, and that exhaust gas does not enter the crankcase on the exhaust stroke.