Originally Posted by
Ex Pres
The thread pitch is the same, it's the profile of the thread that's a little different. One's 55*, the other is cut at 60*. I can be quite sure I've installed an English headset where an Italian would have been "correct", and not noticed.
This is exactly right. English and Italian headsets (and freewheels for that matter) have the same thread diameter and pitch but English threads have a 60° apex angle while Italian threads are 55°. For all practical purposes, making the switch is completely without drama but, as onespeedbikes noted, don't go back and forth.