I have a CR10s PRO that's broken down ... again !

That threaded tube between heating element and the cooling is leaking melted filament big time !

I saw a video where it was said that the threads should be 6mm threats - I think it's an undersize 6mm compared with my 6mm threading tools - it's very loose when screwed together !

What is the right thread size and what can I use to "pack" the threads before screwing them together ?