Sorry about the confusion. The official name of the part in question is "Hot end isolator", photo is for example here.
It was very lose when I pulled out the PTFE coupler, it needed 1-2 turns to be tightened. I am about to assemble everything back but I am not sure if hot end isolator should be so lose or if it should be tight (and how tight).
Ignore my question 2. The space is normal.
you may want to load pictures so people can fully understand your explanations
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