Usually, on aluminum heads (I owned Corvairs in my past, Al Heads) the spark plug gets cross threaded (even at the factory, if you can believe it). The plug appears tight, but over time, it blows out. I"ve seen three engines have this happen.
Heli-Coils work well. Done right, they are stronger than the original threads. On some machinery (Aluminum contruction), the heli-coild are used anyway because of their strength.
Good Luck.
