Intake manifold bolt to nowhere

I would like to add a sending unit for a digital water temp gauge somewhere in the water circuit on my XK 140. There is a bolt just adjacent to the carburator closest to the firewall on the intake manifold that apears to not be doing anything really. It treads into a blank hole. Question is is that the water circuit behind it or is it the air intake? If it is the water circuit could I not drill and tap the hole for 1/8 inch NPT.

That bolt is used for holding the air cleaner on a Mark VII. They did the same thing on later 120 from '51.
Behind it is water, not air intake.
Your idea is interesting. I might try it.
However, I don’t see any difference between measuring the temp inside the head exit water and in the manifold metal. In fact, manifold temperature may be more what you are really interested in monitoring, as it affects air/fuel mixture.
I would not drill through, just find a sender with the right thread or tap blind, and see how it works.

The sender in mind is 1/8 inch BSP thread British standerd pipe thread its a little different than NPT that i am used to. The hole is 3/8 inch the tap takes a 11/32 bit which would leave .40 inch of existing thread. But the taper in the tap makes up for that so not a perfect senario but i think it will seal. I am going to go for it. Will tap then driil small hole to the water jacket. Let you know how it goes.

Be aware that there is BSPP (parallel) and BSPT (tapered) and identify which you have.
Chart attached.
10-1009.pdf (153.8 KB)

The whole ordeal took about 20 minutes to complete once everything was in order. Used a 11/32 bit and a 1/8 inch 2BSP tap. This was the size of the sending unit i purchased from Koso they make small thin digital gauges for motor cycles and that thread is common. The through hole i drilled into the cavity was 25/64 and i would not want to go much larger as the hole is just at floor level of the water jacket. The ribbing between the air intake and water jacket just below.