I'm going to take it in.

 

One more vote in favor of the Heli-coil.

One thing Silver-Bolt will undoubtedly affirm is that you will want to ensure that the proper thread count is used so as to ensure that no part of the coil becomes a protrusion which would open itself up to losing contact with the head and becoming so hot as to become a glow-plug which would cause dieseling/detonation to your piston's peril.

If cost were no object, I'd rather have helicoil inserts in every threaded hole in my block than simple tapped holes -- especially for bolts that were removed & reinstalled frequently (as in a race-engine).

 

We heli-coil military aluminum parts all day long. They work great. I would try and not remove the head if you have decent access to the plug your working on. Another concern, when you drill the hole oversize make sure the chips don't fall into your combustion chamber! Get someone with experience to do this for you. The fix is indeed better than new!

 

Gang:

I would strongly recommend removal of the head. Not only will you be able to do a better job overall (how will you be able to check protrusion with the head in place?), but you'll then be almost duty-bound to do all four.

Additionally, one small chip, and I mean SMALL, stuck to the exhaust valve seat or to the edge of the valve itself is an instant fatality, as far as your engine is concerned. I wouldn't like to see a tiny chip wedged between the piston and cylinder wall (smeared), either.

 

Unless you are trading it in don't even try and install the inserts with the head on. You will get chips in the cylinder and you will not know if the insert is set at the proper depth.

Heli-Coil trivia.

The latest version of the Boeing 747 has over 280,000 Heli-Coils in each plane.