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.

__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250