If its snapped of with spanner pressure, its not going to move with an extractor, so its got to be drilled out. If your a competent mechanic, get a small drill, measure the thread length on an extracted stud, put a bit of tape round the drill at the same measurement so as not to go in to deep. After finding the centre if the stud, get someone to make sure your drill is upright. Go in up to the mark. continue with a larger drill. Of hand I do not know the tapping size of the stud, but find this and you then get the appropriate tapHi guys, in the process of removing both heads & all but one bolt has come off nicely apart from one....which snapped level with the block. Any ideas welcome on how to extract it Cheers Damian .
Been there done that! I got a mate to machine it out on his milling machine, but the corrosion between the bolt and the block that caused it to snap in the first place meant that the hole ended up that little bit too big for a helicoil to be able to hold, so it was either make up an insert, which I didn't fancy much, so he tapped it out slightly larger and made a custom stud for me, which we Locktited in, and providing nobody finds out then I'm in the clear.....
Welding a nut onto a broken stud is very often successful. The heat loosens the corrosion due to the differential expansion and contraction of the parts, and together with a squirt of a penetrating lube, will have you on your way.I have dealt with a similar occurrence years ago. I took a plain steel washer with an I.D slightly smaller than the bolt, then placed it centrally over the snapped bolt. I then plug welded the bolt to the washer, taking care to avoid the block surfaces. Then I welded a 13mm nut to the washer. Let it completely cool down, put a 13mm spanner on the nut and work it backwards and forwards until it can hopefully be removed. Has worked for me in many situations.
Hope this helps.