Sean,
British BSC and ISO threaded headsets have 1"-24 tpi threads. Italian headsets have 25.4mm x 24 tpi threads. Looks the same eh?
British headsets have 60° angle threads while Italian headsets use the obsolete Whitworth (British) Standard 55° angle threads.
The 5° difference in thread angles is not that much especially considering
inaccuracy of bicycle threads in general.
The screwed on steel race is much harder (around 58-60 Rc) than the softer steel in the steering tube so it acts as a thread forming die.
The first time you assemble a mismatched steel headset - British on Italian or Italian or British, you reshape the thread form by around 5°. This has the advantage of work hardening the threads on the steerer and making them sligh tly stronger.
Problems develop when you switch back and forth between different threaded headsets. You risk damaging the threads on the steering tube.
With an aluminum headset the steel threads on the steerer reform the softer aluminum. Switching an aluminum threaded race more than once will definitely
damage the softer threads.
I don't know if Italian threaded headsets are being made anymore. So if you already have a BSC or ISO threaded headset on an Italian threaded bike, it's
best to stick with those threads.
Editorial:
Thomas Jefferson wanted to switch to the Metric system when the US was founded but we chose to continue using British Imperial measurements includi ng fractional portions of a dead Brit king's foot!
As the Industrial Revolution progressed the US and many of the European countries involved in manufacturing developed their own thread standards to protect their national mercantile interests.
Products were designed so that people in less developed countries were manufactured good were exported to, had to go back to the manufacturers for replacement parts thus ensuring that "There will always be an England" and s o on.
The British miss-mash of thread standards (BA, BS, BSC, Whitworth, etc.) was
so disliked by other European nations that in 1936 Hitler passed a law stati ng that nothing with British threads could be imported into Germany!
After WWII the UK, US and Canada hashed out an agreement on thread standards
called the UTS or Unified Thread Standards based on Imperial (inch) measurements.
By the 1960s most nations had switched to or were in the process of adopting
the new ISO (International Standards Organization) metric system of threads.
The US, Burma (Myanmar) and 1 or 2 small African countries are the only nations still using the inch system of measurements.
How the Italian bicycle industry adopted a thread system with metric dimensions for diameters and threads per inch (tpi) for the pitch is beyond my comprehension.
The same thing for why the ISO adopted mostly Brit standards for bicycle threads.
Here's a links with a good explanation and history of threads.
http://en.wikipedia.org/
Chas. Colerich Oakland, CA
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