Originally Posted by
bikingshearer
I've only ever dealt with British, Italian and French headset threading. My understanding is, and my experience has been, that British and Italian headset/steerer threading are ever-so-slightly different but not enough to matter; mix 'n' match your British and Italian headsets and forks to your heart's content. French headset and steerer threading is completely different and is not compatible with British or Italian threading and vice-versa.
I don't know anything about ISO threading, except "ISO" stands for "International Organization for Standardization", trying to "rationalize bicycle design" to make parts interchange more easily among bicycles made in different countries.
"Italian" and "English" threaded headsets have the same thread pitch.
However
- Italian (25.4 x 24F) -> { pitch = 25.4mm x 24 tpi, thread angle=55 }
- English ( 1″ x 24 tpi) -> { pitch = 1" x 24 tpi, thread angle=60 }
British vs Italian:
- different angle (ever-so-slightly different, ~ 5 degree)
That's why they are "practically cross compatible" but it's also generally recommended if you make the change from one to another, you keep the new type thereafter, and don't switch up and back several times.
- Lock Nut (threaded) <---------------
- Locking Washer
- Top Race (threaded) <-------------
- Cage Ball Bearings
- Top Cone
- Bottom Race
- Cage Ball Bearings
- Bottom Cone
Steel is much harder than aluminum and almost acts like a "tapper", but screwing a threaded fork onto an aluminum headset ruins the ridges of { lock nut, top race}, and these things are even worse with steel caps.
So, switching aluminum alloy top adjustable cups and top nuts between British and Italian can wear down the threads!
Now, I haven't understood in what ISO differs, or if it's a kind of guide to convert ITA to ISO threaded steerer