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

pishta said
thread inserts are huge. I doubt they would have the room on a poly, as the drill is a .488 for this.
View attachment 1715879666
Helicoil would be the smallest OD repair or drill and tap for next oversize....(gasp) metric!
View attachment 1715879665
I used these for a LA pump housing on the bottom 2 holes from the oil pan. They worked great but the initial hole is huge.

View attachment 1715879667

I'd use these:
McMaster-Carr

Drill bit is an 'X', or .397 diameter. These won't back out like a coil will, and I've found them easier to install. Need to buy the matching staking tool though to set the keys. It's just a stepped punch, but having the right one is critical.