One option is use of proper HOL file for pipe (Withworth) threaded holes. The other option is to create dimension in the drawing and change dimension text to @O1/8" (instead of default @D), but you loose asociativity with that.
They must be pipe threads, inch sizes are widely used in Europe. While working in France, the first part I designed with pipe threads I looked up the ISO metric pipe thread standard and specified that on the print. The French manufacturing engineer came to me and asked "What is this thread?" They were used to ANSI NPT!