I was looking at this old thread:-
Zodiaco Group Thermosiphon Limiter Configuration
It was locked so I couldn't add a comment. So I have started a new one...

Was this ever resolved?

I am wondering if turning this bolt really DID alter the speed of the thermosiphon...and if so, if the orientation of the bolt was positioned with the slot parallel to the back of the machine, did it allow the group to warm up faster and run a bit hotter than when perpendicular to the back of the machine?

Modifying the size of the rod was shown to have a more measurable and reliable effect, even measured with a scace device:
Modifying thermosiphon flow rate on vintage Faema Levers

The "teflon tape" mod is still reliably keeping my groups idling at different temps 9 months later.

Hi Dominico...thanks for your reply.
So the relative position of the slot in the thermosiphon limiter top-bolt has no influence on temperature?

It didn't make any difference that couldn't also potentially be explained by other means. Also, loosening the screw to change the configuration would cause the nut to leak sometimes which I didn't want to deal with. What I did learn is that restricting the flow of the hot leg of the thermosiphon WILL work (as also evidenced by other machines providing needle valves to tune the thermosiphons) and it didn't take me that long to figure out how much teflon tape to add to give me the same effect. That said, if you are handy enough to install a needle valve that is probably the best way to go.

Also, when tinkering with a thermosiphon, don't forget to watch for stalling. A quick few second flush is all that takes to jumpstart it back up again.