Do I have to remove the metal piece inside the unit? It appears to be holding the red arrow in place at one particular temperature.

No! The metal "spring-like looking thing" is a bi-metal coil that actually makes the needle move. If the thermometer is in an area that is "static" in temperature- then the needle wouldn't/ shouldn't move. If you put the thermometer in the freezer and the needle doesn't go down, or the metal coil is scraping the face, then your unit is defective. I agree with the other reviewer. Not only is this unit "cheap" (in a bad way)- it's also not very accurate. I had to have the first one I received replaced, since the metal strip *wasn't* in the slot of the needle.

can this be adjusted? (calibrated)

The angle deflection on the one I received was low 18%. Really only useful in San Diego. In Minnesota, not so much. I was able to calibrate it, but it will depend on your skill level, motivation and curiosity. Angle deflection is proportional to the length of the bi-metal coil strip and inversely proportional to the thickness of the bi-metal strip. It is not dependent on the number of turns or the radius of the bi-metal coil. I had to reduce the thickness of the strip in order to increase the angular deflection, coil it back up and put it back together and then adjust the length of the strip as I compared it to a lab thermometer for reference at a cool room temperature 60 F, and outside temperature, at the moment 0 F.

Is the back flat so I can mount with double sided tape?

The outer edge is raised about 1/8th inch. in the center is a small ring, part of the plastic back, that is about a 1/4" high. both. In between is flat plastic. these raised areas could easily be filed down. However, there would still need to be a small nib that the thermometer dial is mounted to that sticks up approx 1/8". It could be filed down a small bit but not much. Please try and find a backside photo and think of my suggestions.

Hello men. mine is stuck at 80 degrees. how do i fix this? thank you gents.

Greetings from Filament Brands ! We are sorry to hear that you thermometer is stuck at 80 degrees and apologize for the inconvenience. We happily stand behind our products and this model is backed by a 1 year warranty. To best serve you, we kindly ask that you reach out to us directly by calling 866-843-3905 Monday thru Friday 8am-5pm MST or email us anytime at prodsupport@filamentbrands.com and we will be happy to replace your thermometer under warranty.