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Threaded Headsets
Size
Steerer O.D.
Stem diameter
Steerer I.D.
Crown race
Inside diameter
Frame Cup
Outside Diameter
Threads
Per inch
Notes

BMX/O.P.C

bikes

.833"
(21.15 mm)
26.4 mm 32.7 mm 24 Used mainly on bicycles with one-piece cranks, also some early mountain bikes.
French 25 mm 22 mm 26.5 mm,
27.0 mm
30.2 mm 25.4
(1 mm)
Obsolete. French steerers usually have a flat filed on the back, rather than a grooved keyway as with other threaded systems. Interchangeability note:

These 5 sizes of forks
and headsets
all fit the same frames,

since the head tube
size is the same.

1" ISO Standard
(25.4 mm)
7/8"(22.2 mm) 26.4 mm 30.2 mm 24 This is the standard 1" size.
1" Italian (25.4 mm) 7/8"(22.2 mm) 26.5 mm,
27.0 mm
30.2 mm 24 Obsolete. Threads are cut at 55 degrees, but ISO or J.I.S. headsets can be used.
1" J.I.S.
(25.4 mm)
7/8"(22.2 mm) 27.0 mm 30.0 mm 24 Older or lower-quality bicycles from Asia
1" Raleigh
(25.4 mm)
7/8"(22.2 mm) 26.4 mm 30.2 mm 26 Proprietary size used on Raleighs made in Nottingham, England
Austrian
(26 mm)
22 mm 26.7 mm 30.8 mm 25.4
(1 mm)
Higher quality Austrian bikes use English/ISO
French Tandem 28 mm 22 mm     25.4
(1 mm)
Obsolete and rare.
1 1/8" (28.6 mm) 1" (25.4 mm) 30.0 mm 34.0 mm 26 "Oversized" (This size is more often used for threadless systems.)
1 1/4" (31.8 mm) 1 1/8" (28.6 mm) 33.0 mm 37.0 mm 26 Mainly used on tandems
Threadless Headsets
Size/Stem diameter
Steerer O.D.
Crown race
Inside diameter
Frame Cup
Outside Diameter
Notes
BMX/ 26.4 mm 32.7 mm Used mainly on bicycles with one-piece cranks.
1" ISO Standard (25.4 mm) 26.4 mm 30.2 mm This is the standard 1" size.
1 1/8" (28.6 mm) 30.0 mm 34.0 mm Most newer mountain bikes use this size.
1 1/4" (31.8 mm) 33.0 mm 37.0 mm Mainly used on tandems
1.5" (38.1 mm) 39.8 mm 49.6 mm Proposed OnePointFive standard for downhill and freeride applications.

info from sheldon brown's glossary


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