Stefano as we say in English, there's more than one way to skin a cat! You are doing what I tried to do when I was young. Take an idea and almost make it from the ground up. You are in the right place as that's what we do here! It also helps you have the time and resources to do it.

I think all of us have to rely on the mail for special parts. I know I do. I'm lucky Los Angeles is only 200mi. away and I can get things sent here within a couple of days because there is very little here for machining supplies.

So on pic #2 you are gas welding aluminum horn? And you made the big horns in your other pictures? Are the different sections not just for esthetics? But to keep the horns from resonating or to help them resonate? That has always been the "bastard" for me with horns......they are so COMPLICATED! And my impression is the only way to balance the whole system is to have the same efficiency from top to bottom. And to compound all this the ear hears different frequencies differently. Wow, you have really got a complicated project here.

I would have a really hard time getting big pieces of aluminum like In your photos. If it's so hard to get, and press fit would do instead I'm curious why you still want to thread on the guide? I'm not criticizing, I truly am curious about all this stuff.