Good news! I installed an 1/8" NPT temperature sending unit for a dash mounted CLT gauge in this very location. It threaded in nicely. But just in case I added a little high-temp RTV to the threads of the sending unit before I screwed it all the way in. It threaded in with the same feel as one would expect for an NPT fitting, a slow build up of torque as the threads seat.
I plugged the sending unit into the gauge, started up the car, and let it warm up until the fans came on...
- No leaks
- The dash CLT gauge read right at about 206-208F when the fans came on
I was beginning to question the accuracy of this gauge based on the readings I obtained with the sending unit mounted in other locations. But in this location, the temperature is right on. And, there is no need for an adapter fitting, or even any drilling and tapping.
So, if you have an NB, and are adding a dash coolant gauge, and the gauge sending unit is 1/8" NPT, this is a good option.
FWIW this is the ProSport gauge I am using.
Clear Lens Water Temperature Gauge-52mm