Hi all,

ISO indeed is a way of "splitting the difference" for compatibility. An ISO threaded headset will thread onto either true English (aka BSC) or Italian threads just fine. Similarly, in theory if a fork is threaded ISO either BSC or Italian threaded cups should spread on.just fine. If memory serves me, I've had a framebuilder cut threads on forks and this was the case. And of course, if a steering tube and headset are both threaded ISO then it all works perfectly.

What I have found (and I could have this reversed so I apologize if that's the case - double check me on this) is that BSC cups thread a bit loosely on true Italian fork threads. It's workable, but just make sure the cups are tight when you adjust the headset or you're more likely to have loosening issues and possible thread damage. Now, using Italian threaded cups on BSC threaded forks results in a cup that is really too tight to easily thread on. I suppose you could strong-arm a steel cup onto the threads and change their pitch, but it's really ugly and I don't do it.

Now, for freewheels the story is similar...but use of an Italian freewheel on true BSC hubs results in the freewheel going on a bit loose and I don't think there is any thread damage at all - but a BSC threaded freewheel will go onto an Italian threaded freewheel most of the way and then it takes some force to get it on the rest of the way. So once you "BSCize" the Italian threaded hub, I can easily see why you wouldn't go back. I think it's rare to have issues using Italian threaded freewheels on British hubs. But - if a rider is insanely strong and especially if the hub is on the small side of the tolerance, then a problem is possible. In general, most folks interchange without thinking about it much. And of course, if you have either the hub or the freewheel with ISO threads (or both), it will play perfectly with any of the threads except French.

Mike Kone
Boulder Bicycle
Classic Bicycles Auburn