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Threading Systems
For normal, generic nuts and bolts, there are two threading systems in common use, S.A.E. (U.S.Standard) and Metric (I.S.O.). S.A.E. threads are designated by a diameter and a number of threads per inch (TPI). For example, the common 1/4 - 20 means a bolt 1/4 inch in diameter, with 20 threads per inch. Metric bolts are sized by the diameter and the distance between adjacent threads, for example 6 x 1 refers to a bolt 6 mm in diameter with threads 1 mm apart. (This is the size used for brake mounting bolts and brake shoe hardware.) Similarly, 5 x .8 means a bolt 5 mm in diameter, with threads .8 mm apart. (This size is used for fender and bottle-cage fittings, shift lever bosses, and most cable anchor bolts.) A third system, known as Whitworth, was used in Britain up until the 1960's when the U.K. converted to metric.
Bicycle parts come in even more different thread systems than common nuts and bolts. There are different standards for headsets and bottom brackets for American/BMX/OPC, British, French, Italian, Raleigh and Swiss bicycles.
The table below is for threaded bottom-brackets. For the Ashtabula OPC and other unthreaded bottom brackets, see the unthreaded bottom-bracket cribsheet.
Bottom Bracket Threading
| Standard: | Threading | Adjustable (left) cup/cone direction |
Fixed (right) cup/cone direction |
Shell Width |
Applications/Notes |
|---|---|---|---|---|---|
| British I.S.O. |
1.370" X 24 tpi 1.375" X 24 tpi |
right | left | Standard 68 mm O.S. 73 mm |
The overwhelming majority of bicycles in current production. British and I.S.O. are interchangeable. |
| Shimano Hollowtech II, FSA MegaExo, RaceFace X-type | ISO | right | left | 90 mm, 95 mm including cups | External cups for cartridge bearings fit British/ISO threaded bottom brackets or unthreaded shell. 24 mm spindle, spacer to use 6805 bearings with 25mm I.D.. Bottom bracket shell must be faced so cups are parallel. |
| Campagnolo Ultra-torque | Italian or ISO | Spindle diameter 25mm. | |||
| Chater Lea | 1.450 x 26 TPI | right | left | Obsolete British size but available from Phil Wood & Co. | |
| French | 35 mm X 1mm (25.4 tpi) | right | right (wrong!) |
68 mm | Obsolete, used on older French bicycles. Prone to problems due to the right-threaded fixed cup, which tends to unscrew itself in use. |
| ISIS Overdrive I | 48 x 1.5 mm | right | left | 68mm 100 mm |
|
| ISIS Overdrive II | 48 x 1.5 mm | right | left | 68mm 100 mm |
New proposed standard oversized system. |
| Italian | 36 mm X 24 tpi | right | right (wrong!) |
70 mm | Italian and some high-end French bicycles. Prone to problems due to the right-threaded fixed cup, which tends to unscrew itself in use. |
| Phil Wood American Isis (comments) | 50 mm | right | left | 1.5 mm thread pitch. | |
| Raleigh | 1 3/8" X 26 tpi | right | left | 71 mm 76 mm |
Older British-made Raleighs, 3 speeds and others from the Nottingham factory. |
| Swiss | 35 mm X 1mm (25.4 tpi) | right | left | 68 mm | Same thread as French, but fixed cup is left- threaded for reliability. |
What happens if you try to mix different sizes:
| Bottom Bracket Shell Threading (Below) |
BritishI.S.O. 1.37/1.375" x 24 tpi CUPS R-L (34.8/34.9 x 1.06 mm) |
Italian 36 mm X 24 tpi CUPS R-R (1.417" x 1.06 mm) |
French 35 mm X 1mm CUPS R-R (1.378 x 25.4 tpi) |
Swiss 35 mm X 1mm CUPS R-L (1.378 x 25.4 tpi) |
Raleigh 1 3/8" X 26 tpi CUPS R-L (34.9 x 1.06 mm) |
|---|---|---|---|---|---|
| British/I.S.O. 1.37/1.375" x 24 tpi (34.8/34.9 x 1.06 mm) |
Made to Fit | 36 mm Cup diameter is too large, thread won't even start. | Cup diameter is slightly too large, usually won't start. Right (fixed) cup threaded in the opposite direction. |
Cup diameter is slightly too large, usually won't start. | Diameter matches, but thread pitch does not. Will bind after only a few threads are engaged. |
| Italian 36 mm X 24 tpi (1.417" x 1.06 mm) |
British/I.S.O. cups fall through | Made to Fit | Italian shells are larger diameter, all other size cups fall right through, threads will not engage. | ||
| French 35 mm X 1mm (1.378 x 25.4 tpi) |
35 mm = 1.378". Shell is slightly larger, thread pitch slightly finer. Left side may seem to fit, but will be loose. |
36 mm Cup diameter is too large, thread won't even start. | Made to Fit | Left (adjustable) side is interchangeable. Right (fixed) side is threaded in the opposite direction, won't fit. |
35 mm = 1.378". Shell is slightly larger, thread pitch slightly coarser. Left side may seem to fit, but will be loose. |
| Swiss 35 mm X 1mm (1.378 x 25.4 tpi) |
35 mm = 1.378". Shell is slightly larger, thread pitch slightly finer. May seem to fit, but will be loose. | 36 mm Cup diameter is too large, thread won't even start. | Left (adjustable) side is interchangeable. Right (fixed) side is threaded in the opposite direction, won't fit. |
Made to Fit | 35 mm = 1.378". Shell is slightly larger, thread pitch slightly coarser. May seem to fit, but will be loose. |
| Raleigh 1 3/8" X 26 tpi (34.9 x 0.977 mm) |
Diameter matches, but thread pitch does not. Will bind after only a few threads are engaged. | 36 mm Cup diameter is too large, thread won't even start. | Cup diameter is slightly too large, usually won't start. Right (fixed) cup threaded in the opposite direction. |
Cup diameter is slightly too large, usually won't start. | Made to Fit |
Freewheel Threading
| Type | Inch | Metric |
|---|---|---|
| Italian | 1.378" x 24 tpi | 35 x 1.058 mm |
| ISO | 1.375" x 24 tpi | 34.92 x 1.058 mm |
| British | 1.370" x 24 tpi | 34.80 x 1.058 mm |
| French | 1.366" x 25.4 tpi | 34.7 x 1 mm |
| Metric BMX | 1.181" x 25.4 tpi | 30 x 1 mm |
French and Metric BMX freewheels thread don't work with anything else.
ISO, English and Italian are all semi-interchangeable, but it you shouldn't go back and forth between different types of freewheels on the same hub repeatedly.
Headset Threading
| Threaded Headsets/Steerers in order of frame cup outside diameter, crown race inside diameter, threads per inch, and steerer O.D.-- the dimensions which affect headset compatibility with frame and fork. Crown race seat of fork can be milled smaller, frame bearing seats milled larger. | ||||||
|---|---|---|---|---|---|---|
| Standard | Frame Cup Outside Diameter |
Crown race Inside diameter |
Steerer O.D in mm | Threads Per inch |
Stem diameter, Steerer I.D. |
Notes |
| 1" J.I.S. | 30.0 mm | 27.0 mm | 25.4 mm | 24 | 22.2 mm (7/8") | Older or lower-quality bicycles from Asia. Can be milled to ISO standard. ISO and other 30.2 mm O.D.cups are a very tight fit into the head tube; best mill it to avoid damage. |
| 1" Raleigh | 30.0 mm | 27.0 mm | 25.4 mm | 26 | 22.2 mm (7/8") | Proprietary size used on Raleighs made in Nottingham, England. ISO and other 30.2 mm O.D, frame cups are a very tight fit into the head tube; best mill to avoid damage. |
| 1" ISO Standard | 30.2 mm | 26.4 mm | 25.4 mm | 24 | 22.2 mm (7/8") | This is the standard 1" size. |
| 1" Professional/ Campagnolo |
30.2 mm | 26.4 mm | 25.4 mm | 24 | 22.2 mm (7/8") | Obsolete. Threads are cut at 55 degrees, but an ISO headset can be used. |
| 1" Italian | 30.2 mm | 26.5 mm, 27.0 mm |
25.4 mm | 24 | 22.2 mm (7/8") | Obsolete. Threads are cut at 55 degrees, but an ISO or J.I.S. top headset can be used. Fork can be milled to ISO. |
| French | 30.2 mm | 26.5 mm, 27.0 mm |
25.0 mm | 25.4 (1 mm) |
22.0 mm | Obsolete. French steerers usually have a flat filed on the back, rather than a grooved keyway as with other threaded systems. The bottom headset parts wear faster; you can replace them with ISO parts and use the old top headset. |
| East German (DDR) | 30.6 mm | 26.8 mm (1 1/8") |
26 mm | 26 | 22.0 mm | Very similar to or the same as Austrian. 0.2 mm discrepancy may result from measuring the I.D. of the head tube (press fit). See page on German site. |
| Austrian | 30.8 mm | 26.7 mm | 26 mm | 25.4 (1 mm) |
22.0 mm | These are found on bicycles sold by Sears, made by Steyr in Austria. Higher quality Austrian bikes use English/ISO. |
| BMX/ O.P.C. bikes | 32.6 mm (varies) |
26.4 mm (varies) | 25.4 mm | 24 | 21.15 mm (.833") |
Used mainly on bicycles with one-piece cranks, also some early mountain bikes. |
| French Tandem | Let us know, if you do! | 28 mm | Let us now, if you do! | 25.4 (1 mm) |
22.0 mm | Obsolete and rare. |
| 1 1/8" | 34.0 mm | 30.0 mm | 28.6 mm | 26 | 25.4 mm (1") | "Oversized" (This size is more often used for threadless systems.) |
| Moulton | 30.2 mm top, 36.4 bottom |
33.0 mm | 25.4 mm | 24 | 22.2 mm (7/8") |
Moulton Mark III and earlier. Top headset is standard ISO. |
| 1 1/4" | 37.0 mm | 33.0 mm | 31.8 mm | 26 | 28.6 mm (1 1/8") | Mainly used on tandems |
Metric threads are specified by diameter followed by the thread pitch (distance between threads)
For example, the common "M5" thread used for water bottle mounts, cable anchor bolts, fender/rack eyelets, shifter mounts etc. bicycles is more specifically described as "5.0 x 0.8" which specifies a 5 mm diameter, with threads 0.8 mm apart.
Similarly, the common "M6" thread used for brake mounting bolts, threadless stems, many seatpost bolts and so forth is actually "M6.0 x 1.0" That's 6 mm diameter, threads 1 mm apart.
Normal coarse metric threads are commonly designated with the letter "M" followed by the diameter, with the thread pitch understood. For example:
- M2 = 2.0 x 0.40 mm
- M2.5 = 2.5 x 0.45 mm
- M3 = 3.0 x 0.50 mm
- M3.5 = 3.5 x 0.60 mm
- M4 = 4.0 x 0.70 mm
- M5 = 5.0 x 0.80 mm
- M6 = 6.0 x 1.00 mm
- M8 = 8.0 x 1.25 mm
- M10 = 10.0 x 1.5 mm
- M12 = 12.0 x 1.75 mm
- M14 = 14.0 x 2.00 mm
To give another example, the common size derailer hangers and chainring stack bolts and is 10 x 1.0 mm. This is a fine thread, not a standard coarse thread, so it would be incorrect to refer to this simply as "M10" since the standard pitch for M10 is 1.5 mm.
