I've heard of Ford HD 292 engines having exhaust valve stems that were sodium filled.  Cliff reported in another thread that the stems were ~7/16 in diameter, but I haven't seen any prior to this.  WoW!   

Click the link below to see pictures of the engine for sale on eBay with a cylinder head picture that shows the size of the exhaust valve stems adjacent to the intake valve stems.

http://www.ebay.com/itm/Used-Ford-292-V8-Engine-/112160346844?hash=item1a1d4712dc:g:5OsAAOSwOyJX9~cv


NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York