Heli-coil questions

Yea, my bad, i had an eazy-out on my desk and was staring at it at the time lol. I meant heli-coil.. Well my dad had bought a whole bunch of fittings to plug the extra holes for the temp gauges and so on on his performer manifold for his chevy. So i took one, and drilled the center out and tapped it for 5/16" threads and it turns into the threads i tapped into the head, but i wanted opinions on it first if the heli-coil is to fail.. well ill see soon enough.