Timeserts are pretty awesome - one of the best thread replacements available. They won't loosen since during the installation, the lower threads get flared outwards during installation deforming the outer threads, locking it into the base material. You can also use loctite in addition.
Timesert also makes M11 and M12 head-bolt thread replacement kits and are probably the only helicoil type product I'd intentionally use to reinforce the block as opposed to replacing a damaged thread - the solid carbon steel inserts are stronger than cast iron, and the larger thread diameter reduces the stress on the block for the same bolt preload.