XXL, The fitting you have circled I believe is 1/2 NPT.....there are many variations of that water neck depending upon year,emissions etc.....some years had thermal vacuum switches installed in the water neck for E.G.R. and cannister control. ...there still would only be two options on thread size; 3/8 NPT and 1/2 NPT.....3/8 NPT taps require a .579 drill so are larger than standard 3/8-16 bolt thread.....hope this helps......fats