The .008 exhaust valve clearance associated with '73 and '74 busses is the
  correct valve clearance for any type IV engine that is fitted with sodium
  cooled exhaust valves. Bentley notes that any 1700/1800 that has been 
  retro-fitted with sodium cooled valves should also be set at .008. If the 
  engine has been rebuilt and you're not sure whether it has sodium cooled 
  valves or not, I'd set the exhausts at .008 <greater clearance> just to be
  sure. If you know that normal exhaust valves were used at rebuild time, the
  correct clearance would be .006" for both intake and exhaust.

  The 1800 that I took apart in April <out of my '74> had sodium cooled valves
  and they have a different appearance than a conventional valve. The valve
  stem is tapered outward from the exit of the guide to the head of the valve. 
  It looks like a witch's hat. I think it's because that portion of the valve
  is hollow so that the molten sodium can slosh around and carry heat from the
  valve head up to the stem and guide.

  If you are really curious, that portion of the valve is visible in the 
  exhaust port and could be seen by removing one of the heat exchangers and
  peering up in the exhaust port with a flashlight. 

  Dave Kautz
  dkautz@hpsidms1.sid.hp.com