We install time-serts when we run across this problem. A time-sert, which is stronger than a heli-coil, is a solid piece that gets installed w/ a special tool. The hole is drilled, taped and then counter sunk to accept the insert. When installed properly, the insert is flush w/ the engine case and the stock size drain bolt can be used. Kits are anywhere between $100 to $150 each but well worth the investment for a shop.