[Mk2] [Turbo] Exhaust manifold stud threaded, how much meat around the hole?

[^Trickster^]

Long story short I have helicoiled a exhaust stud a couple of times and they have failed completely after a period of time, now there is pretty much no aluminium left.

I think the next tap size which would give good clear strong threads is going to be m14! I don't have a spare head to see how much meat there is around the hole to its full depth to see if it's safe to do.

I don't really want to have the hole welded up with the engine in the car so would prefer to just go for a stupid size stud if I can get the m14 in.

Anyone gone that far or have a head they could look at for me?

Thanks

Graeme