Have you considered a 5/16-18 "Keensert"? It (Part Number 93340A415 in the McMaster-Carr catalog)calls for a 7/16-14 external thread, which calls for a 23/64" tap drill. Thus you do not need to drill as large a hole for the 3/8" helicoil and still have the 5/16" cap screw for ease of installation. I have had better luck with Keenserts staying put than with helicoils.
