Originally Posted by
OLDYELLR
Over 50 years ago I remember replacing an English freewheel on an English hub with an Italian freewheel (Regina), had no trouble and didn't know the difference at the time. Last year I was looking for an English threaded Regina freewheel and accumulated several, some English, some Italian. When I tried them on my English hub, including some English threaded Japanese ones I had kicking around, I really couldn't feel the difference between any of them. So, at least for English/Italian, I would say don't worry about it.
English and Italian use the same thread pitch, but the thread profile is slightly different. English uses a 55deg angle, while Italian uses a 60deg angle. They are compatible, as Sutherland notes, but some reforming of the threads will occur, particularly with steel freewheel on an aluminum hub. As long as you don't repeatedly switch between English and Italian freewheels on the same hub it should work fine.