Well......... best guess (tricky even under microscope) is 0.16" diameter by 36t.p.i. So possibly 1/8" by 36? Trouble is, there are only 2 or 3 threads visible, so difficult to be 100% sure. Checked with the tables, and none of the standards (Whit., UNC/F or BA) corresponds, as 1/8" is 40 tpi...