itll be fine they tend to come in 1xd or 1.5xd so 10 mm is plenty long enough

I was going to do a kind of 'how to' thing with a bunch of photos, but I made a total horlicks of getting these out today, suffice to say don't try taking photos of the drill spinning in the freshly snapped off stud. You'll break the drillbit and make the job a massive load worse.

its worth noting that the front stud is open to the elements at the back - this is where it corrodes and this shit is dragged through the head as the stud un winds. get in with a scribe and remove as much of it as possible heat it and soak it with diesel a few times , weld the nut on and while still red hot hit it repeatedly with a BFO hammer to break the seal while its still red hot start to wiggle it - dont aim to unscrew it in one go just start the wiggle and diesel treatment wiggle and more diesel and it starts to move

heat diesel and wiggle and i can usually get a stud out with a set of pump pliers and a propane torch - sometimes you just get one thats a pure bitch that wont move ( see Russ's picture above)

I've got the helicoil kit, new studs, brass nuts and the gaskets on their way so I'll do the three that aren't snapped off first and save the best til last. Lol.

I'm so nearly done but the last stud has made me give up until tomorrow. Been at it all day.. I think my drill bits are just blunt now.
I've basically had to drill out one and the other two I just filed flats on the studs and eventually they wiggled free.

Heli-coil is really easy, I always thought it was hard! Or beyond my level of precision at least lol. Two of the holes are repaired and very satisfying that was too. Haven't put studs on them yet though.

Did you remember to wind the helicoil in so that it's one turn below the face of the manifold?

I didn't know that, but funnily enough I thought it'd be a good idea so did do that.

What's the reason for that then?

the coil moves ever so slightly in its placement - (and any threaded hole in a mating surface should be countersunk as the material pulls up under torque) stopping the mating surface from sealing

I've finally finished. It took me over 20 hours though, incredible, just for exhaust studs!

Three of them are lovely, did the helicoil and they torqued up nicely. However the last one snapped off like the others, even after I got them wiggling by filing flats on each side. This one though I ended up stupidly snapping off the HSS drill bit very deep in the stud. Couldn't believe it..
Impossible to get out so I blunted a load of drill bits trying to drill through that. Got cobalt bits to try, still EXTREMELY slow progress, three hours of just drilling. Then the drill slipped off the side of that

All I could think of to do was to put a hole and helicoil it next to the old one. And then I had to make to hole in the manifold match, which was not very hard. The new hole is kind of joined to the old one so I didn't hold out much hope that this horrible bodge was going to work. Actually the first fitting had that coil come out with the stud. So i did it again only much deeper.

Very surprisingly, when I started the car it was whisper quiet! It's like driving a different car. And no more oil leak from the sensor, not a drop to be seen.

Thanks all for the pointers and help.

If it goes again I can weld the hole up and drill a new one for you.