could it be the head is damaged cracked etc near helicoil never heard of a helicoil fitted properly in to a sound metal doing this btw what was blown out old bolt or bolt & helicoil there is a vast difference between a hand tapped thread & a thread tapped on a proper table etc etc . the failed item is the weakest link so on the grounds that the helicoil is stronger than original ( if done properly )then the bolt old or new ? or head must be suspect . hth