I forgot to add that I had tried the two nut trick, heating them to a nice cherry red colour, soaking in freeing oil and what I thought would be the nulcear option of welding a nut to the stud. The problem was that the corrosion was so entrenched that studs were not strong enough to turn without the metal shearing, so no force was ever going to work on the worst ones.

Drilling it out is not that difficult, however it does require that you take it very carefully, in the frame it was really difficult. I find that it much easier to start with a flat surface, ie grind the stud flat, close to the surrounding metal and then punch a mark that is really distinct. Then a small drill bit first, they cut a pilot hole much quicker than using a large one. Lots of lubrication, not too much pressure and it will (usually) work.

If I have to do this again I will put the engine on a bench, I thought it would be straightforward.....how wrong can you be?

Oh by the way never use a stud extractor bit (Eazyout or whatever they are branded) I once tried that on a Lambretta J125 head stud, and it promptly snapped off leaving me with a hardend steel insert....oh how I laughed...

 

Also you could try finding someone that can do spark erosion? Never had it done myself but I know someone who has... Also I have seen a US firm advertising a thread insert which you screw in, so not all is lost if you have to drill out more metal than a helicoil would need, it was on UTube, there are quite a lot of helicoil\thread repair 'how to do it' kind of videos posted on it. Anyway best of luck I hope it works out for you.