Assuming you mean a 2003 Z1000 and the threads that you screw the mirrors to the handle bars, M10 x 1.25. These are the ones that are actually on the mirror mounting stem.
I am working on a mirror extension for my t-100 and I don't have any metric thread gauges. Anyone out there know the thread pitch. The only 10 mm metric bolt I have has a 1.50 pitch and it doesn't work. I am sure it is a 10 mm. None of my 3/8" thread gauges worked.