I'm doing a thread repair for one of my exhaust manifold stud holes. It's a hole that was previously enlarged to 10mm.

I think I didn't drill far enough. I only took out the old threads by hand but it seems the 15mm helicoil needed to go deeper than what was previously drilled. The insertion tool kept spinning, past the tang and into the 8mm hole.

Here is the result:



So close! Just half a thread is out.
What should I do?

I know I can remove it because this is the second one I tried to install. But then what?