Exhaust Crankcase bolt size please?

So, got series 2 casings which don't have the hole under the casings for the exhaust to bolt into and apparantly it's a well known fact that if you don't use this then your bgm clubman will fail, apparantly. Ive never seen anyone advertise these exhausts advertise them as series 3 only but there you go!
So, Im planning on drilling and tapping said block of aluminium but am unsure of the depth of hole, obviously I don't want to go through, but want enough to ensure a good grip. Anyone know offhand?
So, Im planning on drilling and tapping said block of aluminium but am unsure of the depth of hole, obviously I don't want to go through, but want enough to ensure a good grip. Anyone know offhand?