Originally Posted by KarTek

I'll break it down:

1. Helicoils typically work well. The only drawback in a glow plug application is that you have to snap off the insertion tab after the coil is installed in order to allow the GP to go through.

2. The GP seals at the shoulder not the threads so the only concern is the coil being robust enough to hold the GP in place tightly.

3. When I drilled my one bad GP out, I used a "Timesert" which is like a helicoil but it's a solid cylinder that, in my opinion, is a more solid solution.

Hope that helps, ask away if you have any more specific questions.