While it seems that everyone has a complicated solution to this ongoing
problem of cleaning up the threads of where the heli coil once was,
there is an easy answer and it's short and sweet.

Locate a thead size chart, and find the closest next size larger
amreican thread to the metric size. ( I'd tell you but I don't have one
right here in front of me )

And just use a thread chaser in the drain hole.

It'll clean up what's left of the pilot hole for the heli coil that was
there, and give you fresh threads to crank an american thread plug in
the hole.

Your all done in about 10 minutes.