blue beasty
Leaks an prone to bits dropping off
If its flush it should be possible to carefully weld a nut to it, the heat will help as well so long as you dont melt the head!
Sorry my fault its not flush as such I meant not protruding its causing a prob with exhaust manifold letting air in to lambdas so need to get it fixed I am working in Evesham so will have to have a trawl through tinternet. I did look at getting a nut on it but not pos well not for me 
Studs should not be that hard, although they do work harden over time. If you used a proper carbide drill and you can't cut it with that, you ain't going to cut it with anything. Very sharp drill bit slow and lots of pressure is the way to drill hard objects. An electric drill is too fast and will just work harden the metal more. Get some C1150 drills for such jobs they are worth their weight in gold.
Thanks wammers think it was operator error as it was late and I was tired will def get some C1150 drills and have another go
Grit Valve
2000 4.6 Vouge
Try a cobalt drill but that will eat trough whatever the size bolt say 8mm drill a 7.5 hole and retap if no luck you can helicore as suggested.
If he can't cut it with carbide he ain't going to touch it with cobalt. You need a pilot 3.2 or 4 mm before attempting with a tapping drill. Your 7.5 mm is a little on the large size anyway tapping drill for 8x1.25 is 6.8 as i recall.
