Quick Question. Maby someone can help me on this on. WHAT is the fret size of an 8mm head studs

Is it 1.25 or 1.5. Why i'm asking is, i'm busy building a turbo motor with AS41 casing and this casing had 14.2mm inserts with 10mm heads studs. I'm busy making new inserts out of hi ten steel with 8mm fret in side for 8mm head studs, but before i start tapping, i want to know what size fret it is. BROBLEM is, i don't have a 8mm stud laing around this side now.
