TTi headers oxygen sensor thread size.

I have part number 363A headers with oxygen sensor bungs. I need to plug them but I have no idea what the tread size is. I prefer not to weld them shut. I measured the inner diameter to be 16 mm and thread "rise" to be 1.75 in millimeter system. But such thread does not exist claim all shops I have visited. So it might be some inch system thread and I have no tools to figure that out. So does anyone here know the thread size? TTi has no email on their website. Fax is long gone here and paper mail takes ages.