[6pack] egr port

[acs25m at swbell.net]

Hi all,
       I have a question that I hope someone can give me an answer to. I had to replace the head on my TR250 with a later model head. It has an  egr port in it that I need to plug. Does any one out there know what size plug will fit. It looks to be a fine thread. Somewhere between 1/2" and 3/4" or possibly a metric? Any idea where I can find one?
  Thanks in advance,
  John Maneke
  68TR250
  cd3416