Cobra lower Manifold..Coolant Plug size? part number?

Im installing my cobra lower intake manifold. I seem to have lost a bolt/plug that screws in the rear passenger side of the manifold coolant passage. It had a 4 point, square head on it. Does anyone know the part number or size/ thread pitch for this bolt/plug. I think its 9/16th but im not sure. I figure if I get the size and pitch I could just go to the HWstore.

Thanks
geoff

"I got it one piece at a time, and it wont cost me dime."