I have used this method on the broken glowplugs on my M57.Its now i wish i had a lathe so i could make a tool to fit perfectly.
I have the same experience as you with centre punching - it just always goes wrong!!
Can you put the manifold back on and drill with a sharp 8-8.8 (whatever fits the manifold hole) to get a center mark in the stud
Manifold holes are usually quite generous and do not exactly line up with the bolt pattern. Possibly because of expansionJust waiting for the manifold to come back, then I can use it as a guide. My friend, who is skimming it) is going to put a hole down the middle of an M8x1.25 bolt that might also help as there is just (only) enough thread there to get the tip of a bolt to grip. It might be that with the manifold in place both together will hold so that i can get a centre hole started.
What i am not sure about is whether the manifold holes will be in the same place after the warp. We will see.
Noone has suggested, so i suspect its not worth it/will cause damage to try, to weld a bolt onto that broken stud. I'd have to build the weld up out of the head to do this so i imagine i will end up with a mess if i try and probably little success (in my hands!).







