JOB DONE!
I turned my machine upside down to unscrew it. I put some olive oil to help "lubricate", but I don't know if it helped...I asked the help of my lovely wife to hold on tight the machine while I was forcing with a 2$ CAD and a clamp. It worked!!! Now the water seems to distribute better in the showerscreen, as I avoid to tighten very strong...
thanks



