If you're talking about the Thermal Fan Switch in the driver's side radiator tank (sounds like it - two wires, about a 1" hex?) then the threads should be M20-1.5.
If you need to know to merely plug the hole after doing some conversion or other, you can just leave that plug in place and clip the wiring off. If you're doing something else, ask away and we'll see what else we can come up with for you!
"recon" (sic - reckon)(tm) "hihgly"(tm) "seceed"(tm)
"Outback AIDS - Alcohol-Induced Dizzy Spells"