I looked in a temp switch today
 

I've been thinking about a good place to put the temp sensor for my electric fan controller, and one idea was to drill a hole near the edge of the existing factory fan control temp switch and put the probe in there instead of sticking it between fins in the radiator. Switch was a 75C version of the Wahler brand due to its low price and being handy.

I picked a likely location and drilled a single 1/8" hole for the probe. The drill went through maybe 1/4" easy, then no resistance at all. I did a bit of looking, and it was an open chamber, so I drilled a few more holes, then Greg took off both ends on the lathe and took it completely apart.

Short answer is the darn thing is mostly air and it didn't seem like there was anyway "good" to mount the sensor probe so that it would get heat transfered to it. Electrical connector end was hollow with a spring switch mechanism in the middle, brass walls too thin to drill even 1/8" hole directly in, and the "bulb" end was a sealed pressure chamber with what looked like some kind of "wax" in it.

I suppose I could drill a hole in the electrical connection end near the side, place it on its side with the hole down and squirt in a bit of epoxy, then put in my sensor and let it cure and hope that provided enough of a thermal path through the side of the back of the plug, but sticking it between some fins should work just as well. I'll find out tomorrow.