Originally Posted by
macstuff
"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)
"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 }