I know this subject has been covered before but i'm looking for a bit more info.

I have a dreaded broken stud. Tried extractors etc but had no luck. In the end I have drilled up 6.8mm and tapped M8 and managed to get a cap screw in as a temp measure.

Possibility of a problem though. When using the final drill there was Aluminium swarf as well as mild steel from the screw, so i'm guessing I was slightly off centre. My question is how deep and wide can you go before breaking through into anything. I'm thinking of drilling out and Helicoiling or increasing size up to M10 but daren't do it untill I know how much available material there is.

The M8 Cylinder Head Exhaust Manifold Stud Tapping is 24mm deep, and I wouldn't chance going any deeper. There is more than enough metal around the tapping to increase to M10, but replacing the stud with a higher tensile M8 stud (such as JE Engineering) with a Helicoil if necessary would be your better bet.

Stainless said:

The M8 Cylinder Head Exhaust Manifold Stud Tapping is 24mm deep, and I wouldn't chance going any deeper. There is more than enough metal around the tapping to increase to M10, but replacing the stud with a higher tensile M8 stud (such as JE Engineering) with a Helicoil if necessary would be your better bet.

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Thanks for that info, good to know I could go up in diameter.

I managed to drill out, Helicoil and fit a 12.9 stud without removing the Manifold. In the new year I will be fitting a new downpipe so will whip the manifold off, grind the webs and helicoil the rest I think.