rear head coolant plug question
 

I have the transmission out of my '99 right now doing a clutch swap. Since it is very easy to access as a result, I removed a small plug (allen head) that is located on the coolant protrusion on the rear of the head. When looking from above standing on the left side of the engine bay, it is below and to the left of where the heater core hose is connected.

The plug is very small in diameter, and only about 1/4" long. I used a 4mm allen wrench to unscrew it IIRC. A bit of coolant came out when it was removed. The plug seems to thread nicely into a female 1/8" NPT fitting that I have. I have an aftermarket temp sending unit I want to install here. If it is indeed 1/8" NPT then I am all set as I can just screw in the sending unit in its place.

I have searched here and at m.n and cannot find the answer. Does anyone know what the thread type is on this plug?

Here is a picture taken of the spot where the plug goes. This is looking at the left side of the motor. The CAS is visible to the right.

no idea but that's where us NA owners get the signal for our factory coolant gauge

OK makes sense. It has the look of something that was needed before but is not needed anymore.

I think it is an 1/8" NPT but two things give me pause. One, it is a Japanese motor. Two, I remember reading somewhere there is a thread type (metric?) that is very close to a 1/8" NPT but will leak if an NPT fitting is used in it.

UPDATE:

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

Thanks for the update.

Guess i'll be installing my oil and coolant sending units this weekend then :)

Do it! :)

FWIW the two other spots I tried for the sending unit were:
1. Teed into the coolant return hose for the heater core circuit, downstream of the heater core. This read at least 10 degrees cool IIRC.
2. Threaded into a hole I tapped at the exit of the thermostat housing where the upper radiator hose attaches. This one was also not that great, even with a hole drilled in the thermostat to create some continuous flow. It also read cool compared to the stock sensor.

This new spot is basically right next to where the stock NB combo sensor is located. Thus it agrees very closely with the stock sensor based on the radiator fan turn-on point as mentioned previously. It is also the hottest spot out of the three, which is preferable of course.