Colin,
Jason is quite right that an unspecified metric thread can be assumed to be a coarse series one.
However, I strongly disagree with his suggestion to use a 6.8 mm drill. This will give you over 90% thread engagement and you will probably break the tap (or your wrist!) if you try to do it by hand. I just tapped 3 M8x1.25 threads in a 10 mm thick MS plate today, and I used a 7.1 mm tapping drill, which was plenty hard enough. This gives about 65% thread engagement, and the thread flanks will still be about 2.5 times stronger than the core of the screw.
Do yourself a big, big favour and buy a copy of “Drills, Taps and Dies” by Tubal Cain, No 12 in the Workshop Practice series. It’s only about six quid and it will tell you almost anything you are likely to want to know on the subject. The tapping drill tables at the back are worth it alone.
For some strange reason, almost all other standard reference works give ludicrously small tapping size holes for metric coarse threads; these give 85-100% thread engagement and are probably aimed at industrial users, with sharp taps and torque-controlled machines. Machinery’s Handbook even admits that aiming for these levels of engagement is unnecessary and risks breaking the tap, and that 60-70% is perfectly good, and then goes on to reproduce the same old table which does the exact opposite.
One of the reasons for this is that ISO metric taps have a diameter greater than the nominal figure (by about 11% of the thread pitch). Most other thread forms do not have this feature, and the commonly quoted tapping sizes are not quite so ridiculously small, though they are mostly still tighter than they need to be.
David
Edited By David Littlewood on 22/11/2011 19:47:52