In this photo I'm measuring the temperature of the upper thermostat housing.
I know the red laser dot is off slightly from my dot but holding the DSLR camera, with a hot shoe flash, and the infrared while upside down and contorted leaves it up to "close as it gets".
The petcock mounted to the top of this housing is where you would bleed any trapped air off the sealed or anti-freeze side of the system to prevent an air lock. Some engines have this feature & some don't. On some engines this petcock is just in a different location.
*WARNING: Before attempting a bleed a petcock moisten a rag with COLD water so you don't burn your fingers and wear insulated fishing gloves made of rubber. The antifreeze can burn you if you are not careful. I recommend a cold rag over the petcock, rather than pliers because it can spray out at a high force. Trust me, you don't want to use just pliers with no rag to prevent spray while upside down and contorted. Ouch!