Education on NPT Threads

[tmsmini]

With the MPi engines that we are using, I had a custom thermostat sandwich plate made. It was made by a fellow Mini enthusiast.
Originally it was drilled and tapped for the straight threads of the sensors I was using. Originally I was using a mechanical Smiths gauge, but have since moved on to an electric/electronic Smiths gauge and the plate is using NPT thread adapters.

My question is on the tapping of the NPT holes. The sandwich plate has two sensors, one for the gauge and one for the ECU. How far does an NPT threaded male piece need to engage the female?

One of the openings is threaded more deeply than the other and the shorter NPT adapter does not tighten before bottoming out on the lip. How do you measure how deep to the tap the the hole?

[MiniDave]

Could a few turns of teflon tape make up the difference enough for it to seal? You don't need a lot of tight on pipe threads to get them to seal, but you don't want it to work loose either....

I use 2/3 as a guide - like the lower fitting in your middle pic

[tmsmini]

When I switched over to the non-mechanical gauges, the people at Nisonger said to be sure not to use teflon tape. It seems to be one of those issues where there are strong opinions both ways.

I also have a similar adapter on the oil temp gauge. One one of the cars I used a liquid thread sealer(not locker) to stop a constant drip from the oil temp sensor. It seems to be working to seal and has not caused an issue with temp readings.

I need to have a sandwich made for a engine that I a having rebuilt. It will go on the test stand to play with some FI variations.

[MiniDave]

I could see it potentially (see what I did there?) causing issues with electrically controlled gauges, but I would need someone to explain why it would matter on a mechanical gauge, where the contact with the water and temp indication is done by the bulb in the water, not contact thru the threads.

[MPlayle]

The "do not use teflon tape" recommendation is due to the nature of many of the electric/electronic sensors - the sensor body is the "ground" connection and thus needs a good ground path.  The teflon tape forms an insulated barrier, reducing the ground connection.

The mechanical gauges use an "liquid" filled capillary to drive the gauge and do not need the electrical ground.  They can have teflon tape used to assist the thread sealing because they do not need the electrical ground.

[BruceK]

I installed a mechanical capillary-type temperature gauge on my Toyota truck (the stock electronic temp gauge works sporadically – maybe a bad ground?).  I had to use a NPT tap to cut into the temperature gauge sender adapter I installed inline on the top radiator hose.  But I did not use Teflon tape, I used this thread sealant instead and it provided a nice seal with the NPT threads.

[tmsmini]

It was when I made the switch to the electric/electronic gauge that he said no tape.
I think I used some of that sealant on my son's El Camino when we switched out the dash.
I need to dig around for it.

Some EFI temp sensors use two wires where they provide a separate ground.