[embee]

Just an addition to Mark's excellent concise description.

Helicoil is a trade name, though perhaps one of the best known and generic for thread inserts, but there are other makes. I have some by "Recoil" and also "V-coil".

The kits are quite expensive; drill, tap, inserts and fitting tools. You'll probably only need M6x1.0 and M8x1.25 for most general use, spark plug ones you usually only get if you have a specific need for it. They also come in different lengths, 1xD is often sufficient since it is significantly stronger than the parent alloy engine case material.

There are subtly different inserts, some can have a locking feature (usually a straight section across a chord), and they may or may not have break-off tangs (notched ends which break off when turned backwards or tapped through). Make sure you collect the broken-off tangs.