Sodium-cooled valve

An exhaust valve with a hollow stem that is partially filled with metallic sodium. When the valve gets hot enough (138°C), the sodium melts and moves up and down in the valve stem. The sodium absorbs heat from the hot valve head and carries it away to the outer part of the stem and then into the valve guide in the cylinder head and next to the cylinder head itself. This circulation cools the valve head and thus the valve is allowed to run at a lower temperature.