The valve stems are not completely filled with sodium. As the valve heats
up, sodium vapourizes and rises up the valve stem by convection. This
carries the heat up to the top of the stem where it is dissipated by the oil
and air surrounding it. The sodium condenses and falls back to the bottom of
the valve and collects heat from the seat and continues the cycle. It
effectively forms a "heat pipe" to transfer the heat away from the seat .

Tom

"Artemia Salina" wrote in message
news
Dumb question I guess, but why do engine manufacturers make
valve stems that are filled with sodium (I think only exhaust
valves are sodium filled). Is it a heat expansion issue?

Thanks.

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