On Sun, 06 Mar 2005 18:28:02 -0500, Jeff Wisnia wrote:

I recall first hearing about sodium filled valve stems in relation to IC
aircraft engines and some racing cars. I just did a little Googling.

Further supporting the theory that "You've got to take the bad with the
good", I found this text related to aircraft engines which claims that
sodium cooling the valve heads can accelerate valve guide wear.

Saab engines typically do many hundreds of thousands of miles, and I've
never done or even heard of anyone having to redo the valve guides.