2008 OM642 in Mercedes R320.

I imagine the engine had black death at one point on #2. Someone tried to remove the injector hold down bolt and it broke off. Rather than removing the broken bit, they helicoiled the top portion of the hole for a 5/16" bolt (about 8 mm). The engagement wasn't great, so they used JB weld to help seal/hold the injector. Or at least that's my theory.

I've had no luck with a small slide hammer so I have a bigger one. And I'll put some heat to it. Once the injector is out, I will work on hole repair. The bolt wasn't that tight so I think the JB weld is largely holding it in place and will need heat to release.

I now have a short 5/16" bolt with a pilot hole. I'll screw the bolt into the helicoil, and then use the bolt to guide left-handed drill bit to drill into the broken M6 and start the extraction process.

Once it's all said and done, I have a feeling I'll end up with a Timesert.

I assume the M6 hole in the head is threaded right from the surface? How deep is it approximately?

If I replace the threads with steel, then 10mm of engagement should be sufficient. If the threads start at the same spot, then the distance from the first thread to the clamp will be the same, so the clamp load/spring should be the same as OEM. Mercedes added an extra 90 degrees of torque to the bolts so it wasn't very well engineered from the start anyway.