I'm pretty sure the "stem" is just the 1/4" diameter probe section.
Amazingly Miljoco doesn't have a decent engineering drawing but where it mentions the stem length it specifies 1/4" diameter.
Here's a view from above:
View attachment 415449
The long side of the tee is 2.125" overall length, there's about 1/8"" of the pc riser showing and about 1/4" of the thermometer threads showing.
That would put the tip of the probe somewhere inside the pc riser.
btw, you can see how close the ports are side-by-side...
Cheers! (it was hella hot here today as well. It's been awhile since I've had to run the A/C this late in the year)