Did look up the drill & tap charts found on 'net.

I did find a store that supply 37/64" drill for 3/8-18 NPT tap but boy
that is only an HS drill costs 30 CDN !? (snap-on canada) but does
have both taps there but no 45/64" drill for 1/2-14 NPT tap. Sheesh.

The plan with these is occasional job. Like cleaning up 3/8 NPT
thread on a engine head aluminum is very soft. Other times have to
make a brass threaded sleeve with 3/8" female thread out of male brass
1/2 NPT plug and drill out the ruined plug hole (this happened already
to mine and in process asking here now) to size and tap with 1/2-14.
This 3/8-18 NPT hole is used either for coolant temp sensor or a 3/8
NPT plug, but this location also is useful for purging air out of
cooling system & filling with coolant after any service that required
coolant drained. I can anneal the brass plug with propane torch or
stove top coil glowing orange for a short time.

But I wonder is this possible to use available drill bits (1/2" for
3/8-18 NPT tap in annealed brass and 1/2-14 with bit larger drill bit
for aluminum) and really go slow & gentle and frequent backing up on
tapping with plenty of cutting oil?

Depth is around 50% for 3/8-18 otherwise any deeper the tip of sensor
would hit the thermostat. Doesn't matter for the brass sleeve because
I will cut it to length after drilling & 3/8-18 female internal thread
cut a 1/2-14 NPT plug.

The key is to get these taps within canada and hopefully use readily
available drill sizes.

The key is to get water-tight seal using telfon tape, not for
precision job.

Merry-xmas & Cheers,

Wizard (Jason Pero)