M16x1.5mm thread. you can buy the M16x1.5mm to 1/8npt thread adapters at Supercheap Auto, part number is CAL320055 and they are the "Calibre" brand. You'll find them on page 9 of the catalogue: http://media.superch...erformance.pdf.
ps. there is a spare M16x1.5 port in the external thermostat housing that has a bung in it, that's where I put aftermarket temp gauge sensor, that way you have both temp gauges incase one stuffs up.
Have a look at my thread, about halfway down the page you'll find what I'm talking about: http://www.rollaclub...d/page__st__315