hi,
I am a bit confused.
I am looking for a 1" headset, and I looked at Velobase about the Tange-Seiki Levin CDS, which is reported with an available threading

• English
• ISO
• Italian

is ISO threading compatible with English threading?
are they the same?

The threading will work, you just need to know the crown race diameter to properly fit on your fork

Is ISO threading the same as English threading?

yes. But, English/ISO headsets are available in ISO and JIS sizes, which are not compatible. The crown race ID on JIS is 27.0 mm and on ISO it's 26.4. Measure your fork before you buy.

I'm not sure what ISO threading means on a headset. If "ISO" in this context is the same as "French," it is not compatible with British threading.

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.

As has been mentioned, you do have to pay attention to the crown race diameters, as those can and do vary by not a lot, but enough to be require some annoying filing or finding a lower press-in cup from a from a different headset,

Discussed here:
Italian threaded fork

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

I've used Tange-Seiki Levin CDS (JIS) on a 1982 Holdsworth Avanti and a 1961 Holdsworth Cyclone, which I'm guessing need English Threading (but JIS race diameter).

Found the last order:
https://www.spacycles.co.uk/products...&Submit=SEARCH
No specification of threading on there.

Stack height 33.3mm which is quite short.
Just make sure if you need JIS or not.

how confusing it goes ...

I think the upside from that is you just need an accurate race diameter unless you have French / Italian.
27.0 -> JIS
26.4 -> English (BC1)

When I've bought headsets it always says 26.4 or 27.0 + "did you really want JIS".

Here's the full list of headset sizes: https://www.sheldonbrown.com/cribsheet-headsets.html
I make that 13 "standards".

I have a couple of Rudelli headsets on a of bicycles { Coppi 1997, Moser Forma/96 } that use Italian-threading and standard forks.
But the good news, the handcrafted NivaCrMo fork and the headset that I want to get for my new titatanium bicycle, are both "ISO standard", with a declared "crown travel 26.4mm".

Good!!!

Just, to makes thinks more complicated, if the headset manufactor's website says "26.4mm", and you you have to use your immagination to guess to what that number is referring to.
"crown travel", you guess, but when you want to doubple check in the reseller's description ... well you found nothing but "ECxxx".

What is ECxxx?!?

The Customer Service explained that the crown race are the bearings at the bottom next to the fork

EC26.4 -> External Cup, crown travel=26.4mm

so, "ECxxx" refers to External Cup (vintage headset), while the xxx number refers to the inside diameter of the headtube.
However these are not actual measurements they are a way of classifying headset groups.

Thank you for the info - good stuff.

FWIW, I mentioned exactly this to a very knowledgeable long-time frame builder and general bike mechanic; he said his experience was that this wasn't a problem in practice. I am not disagreeing with you, just pointing out that it is not a universally-held opinion.

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

BC1 is not equal to BSC/1"

see here, "all sizes 1⁄4 inch and larger use 26 tpi, making them similar to 1 mm ISO, which are 25.4 per inch and also run at a 60 degrees angle"

BC1" -> 25TPI, 60 degrees
BSC/1" -> 26TPI, 60 degrees

Almost - typical BSC threads on a high-end bicycle headset are 24 tpi - 26tpi is used on headsets such as those for many Raleigh bicycles. Both Italian and BSC high quality headsets are 24tpi. The difference between high quality Italian and BSC is the thread pitch.

Just looked at my trusty Sutherlands manual to make sure I'm not having a brain melt....

This is actually how I choose what kind of bikes I like, and which side of the room to keep them on. By which threading their parts are assembled with.
Gotta keep Em' separated. Separate tool box for each also. That stand with the dust on it only holds "French" threaded bikes.

The Caveat Application Table or CAT (as in; "like a cats ass")
Other acronym definitions are also used...

"International Standards Organization"
"Society of American Engineers"
"British Cycle Thread"
"British Cycle #1"
"British Standard Cycle"

Imperial/English/SBT/BSC/Standard/ISO = SAE 26tpi (actually 3 different pitches [course, standard, fine] but 2 of them not used on bikes)
French and Sometimes Italian but not always French = Metric 27tpi
BC1 25tpi (defunct and interchangeable with colorful language)
BCT/Whetworth 'say it sideways' 28tpi (my Fav) Cant use a wrench on these, only a "spanner" will do. (defunct and only for the age'd purist)

Just for thread pitches, then there are steerer diameters, head and cup diameters.
Austrian? German? maybe Belgian? just Fuggetabout it.
What about Russian or Czecholsovolsko? are you sure you want that bike?

Such a mess. Second only to the correlation between ISO, Metric and SAE bicycle tire sizes.
Where 1-3/4" does not equal 1.75" 700-650a/b/c? 635? S2 through S8 EA1/2/3/4?. 3spd...Balloon - 29'r?

I've gotta go sit down.

Not to mention US wire gauges for spokes use smaller numbers for thicker spokes, while French spoke gauges us larger numbers for thicker spokes, and the numbers cross over right in the thicknesses commonly used with bicycles.

Source: Sutherland's 4th Edition

"BC1" is what I found in 80s and early 90s Campagnolo headsets.
E.g. on my Victory headset. On the back of the cup, it says "1inc 25TPI", while the original box says "BC1" threading.

So, back to the original question, diff(ISO,BC1)= { ISO is 24TPI, BC1 is 25TPI }