To measure the effective diameter of an external metric thread (included angle is $$60$$ degrees) of $$3.5$$ $$mm$$ pitch, a cylindrical standard of $$30.5$$ $$mm$$ diameter and two wires of $$2$$ $$mm$$ diameter each are used. The micrometer reading over the standard and over the wires are $$16.532$$ $$mm$$ and $$15.398$$ $$mm$$ respectively. The effective diameter (in $$mm$$) of the thread is