I’ve used this product for years , to seal the head studs in a flat head Ford V8 to the intake bolts on a Chev intake that go through into the water jackets

 

Go get pipe dope.
In my opinion don't use red or blue loctite
Just to let you know, to use loctite correctly both thread surfaces would need to be clean and dry. Also loctite (red or blue) is a liquid epoxy and red is stronger than the blue.
The dope will seal the threads and it will be much easier to remove the sensor if you have to.
Or like RJ said get some teflon in paste form.

 

Thanks for everyones detailed help, I provably with have made it without. Synopsis to follow, read if you wish.

I finished the coolant flush. I an not mechanically experienced, so the whole op took 11 hours of work. 4 that was easily spent tinkering and fiddling.

I was able to remove and also secure the water pump bolt without taking the off, for which I had no tools.

I had the car securely up on two jackstands and apoeiach from the front underside of the car. I was able to connect three separate wobble socket extensions to attach the 13mm socket to the bolt head with a little wiggling to mate them. To secure it back, I had to reach from behind the passenger side wheel and use my fingers to secure the threading. Then I came back to the front to tighten with the wobbles. Before I filled the radiator 50/50 dexcool, I added a thin layer of pipe joint paste with Teflon to the threads nearest to the bolthead. I didn't add any higher than halfway up threaded segment.

Most of my time was spent switching extentions, and waiting for the engine to cool. I ended up needing lots of shop towels, shop rags, extra empty gallon bottles, a Pyrex measuring cup, tubing, and a good night's sleep. That one is still to come.

Water pump bolt (18 ft lbs)
13mm short socket
6 in wobble socket drive extention
3 in
1.5 in
5/8 driver

Radiator
Petc0ck socket, carbon fiber, moded.
3in drive ex
0.25 driver

Top left T-stat housing bolt (89 in lbs)
Small hands
10mm socket
0.25 in driver

Bottom right T-stat housing bolt (89 in lbs)
0.25 universal joint
6in extention
Converter
0.25 in driver


Tools for coolant flush and Tstat replacement



Tstat inner detail

Left new from amazon Adelco store. Right is damaged from car

 

Thank you for the follow up now burp the system and enjoy the heat!
https://www.chevyhhr.net/forums/how-...-system-60614/

nothing complicated just drive the HHR let us warm up fully then park it with the front right higher then the rear bumper, either park in a hill or jack up the passengers side front .
Let it cool over night and open the filler cap and top up the Dexcool 50/50.
I suggest doing this at least 3 times.