rethreading thermostat holes from 5/16 to 3/8 ?

YY1 said
What size drill for a 5/16 coil?

What size for a 3/8 tap?

Drill for a 3/8-16 is 5/16 (.3125).
Drill size for a 5/16 helicoil is 'Q', or .332.