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I am trying to install a front wheel on a bike - the axle of the wheel is threaded, but I'm having trouble finding out what size thread it is, and what nut can go on this. The size of the thread on the axle is the same as an 8 mm screw, but a standard M8 nut (with 13 mm hex) does not fit on it. Do bikes have some special kind of nut there?

If I look at the thread, and at a standard M8 screw, the thread on the bike axle is a bit more dense then on the screw, but I am not aware of existence of two kinds of M8 nuts, so far every M8 nut I've seen fits every M8 screw.

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    For those coming here looking for a wrench size for the bolt, it's probably 15 mm. – Matt Zukowski Jul 24 '13 at 15:30
  • It depends on the bike. Most bikes bought in a bike shop made by any large brand use mm, but some cheaper bikes purchased at big box stores use american or non standard bolt sizes. Even on bikes that use the metric system, 5mm bolts are very common, so are 8mm and even smaller ones. – sevargdcg Jul 24 '13 at 22:35
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    Simplest thing to do is to take the wheel (or just the other nut) to a hardware store and have them find a match in their bins. – Daniel R Hicks Nov 23 '13 at 19:46
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    I was looking for the same. Took wheel with axle to hardware store, but nothing matched up with the threads. It appears to be extra fine thread. If I used what they had, I would've had to force the nut onto the threaded hub shaft and it would've been ruined. I'm going to have to go to a specialty fastener shop or contact the manufacturer representative, if possible. Seems odd that they – user19415 May 10 '15 at 15:13
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There's a wide variety of axle diameters and threading.

There is a spiffy table here: list of common bicycle thread sizes from Park Tool.

  • Thanks, so it seems like bike axle thread has a pitch of 1 mm, while a standard M8 screw/nut has a pitch 1.25 mm. – miernik Nov 5 '10 at 17:21
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    the link appears to be broken – RoboKaren Aug 2 '15 at 4:36
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    Answers should include answers, not just links. – jqning Nov 8 '15 at 4:21
  • @mikeeng I have rejected the edit, I think it would be better to fix the link if possible instead of removing it entirely, maybe the information still exists but has moved. However, it shows the problem with link only answers and a better solution would be to ask the author to improve their answer and/or flag it for moderator attention (as being a link-only answer) – Swifty Apr 30 at 16:57
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Park Tool list a number of common sizes.

3/8 inch x 24 tpi Some solid axle bikes, including coaster brake
3/8 inch x 26 tpi Solid rear axle
10mm x 1mm Most quick release rear axles
10mm x 26 tpi Rear axle, quick release, Campganolo®

Note that the difference between 24 and 26 tpi is small and can be subtle, meaning that if you're using a spanner to do the nut up you can wreck the nut without really noticing that anything is wrong. The symptom is often that the nut strips before ever becoming tight. To avoid this put the nut on by hand until it is right over the axle. If it still spins freely it is the right thread pitch.

Less commonly you will find 12mm or 14mm axles, often on load bikes and Asian or African bikes, and some mountain bikes. But those are fairly obvious. Some older front axles were 9mm or even 8mm, and they bend easily. It's sometimes possible to replace a 9mm axle with a 10mm one, depending on the bearing cups you have available.

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Could be 5/16" which is very close to M8 (7.9375mm vs. 8mm). Thread might be UNF 24 (24 threads per inch) or British Cycle Std. at 26 threads per inch. There are even some that are metric in terms of diameter but use BSC for thread spacing! If it's 5/16" at 26 threads per inch then an M8-1.00 nut may fit since 25.4 tpi is very close to 26 tpi and 7.9mm diameter is very close to 8mm.

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It is a M8 1.0 nut. I found out the hard way because all the hardware store has was a M8 1.25, so I bought it and the threads were way too coarse

1

First try your LBS, take the wheel with you.

You might save yourself pain in the long run by swapping out the axle for a standard size thread.

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Assuming that the thread on the axle that you are trying to fit a nut is metric, you need to be aware that with metric threads there are 3 different pitches on every size. So, in your case you need to know if the threads on your axle are 8mm course, 8mm fine or 8mm extra fine.

Also as already mentioned, your axle could be an imperial size not metric, for example 3/8" is a common size

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The below answer is copied from ( http://www.parktool.com/blog/repair-help/basic-thread-concepts )

Bicycle Industry Threads

The bicycle industry has a long history of using many different thread standards. Both factional and metric sizes are in use. Some threads are also used almost exclusively in the bicycle industry. Below is a table of some of the threads and their uses. This table is not intended to be complete and exhaustive. Always measure diameter and pitch when possible to determine threading.

Nominal Thread Size     Example of Bicycle Uses
2.2mm x 56 tpi  Common 2mm spoke threading
3mm x 0.5mm     Dropout adjustment screws, some derailleur hardware, accessory hardware
4mm x 0.7mm     Some derailleur limit screws (DIN standard)
4mm x 0.75mm    Common derailleur limit screw (JIS standard)
5mm x 0.8mm     Many uses on bicycles, including derailleur wire pinch bolts/nuts, disc rotor mounting bolts, fender and racks mounts, water bottle cage bolts, and others
6mm x 1mm   Many uses on bicycles, including brake caliper mounting bolts, brake pad bolts/nuts, some fender racks, some brake adjusting barrels
7mm x 1mm   Some handlebar binder bolts
5/16 inch x 24 tpi  Front hubs, solid axle, less expensive bikes
8mm x 1mm   Square-type crank bolts, front solid axle hubs, suspension system hardware
8mm x 1.25mm    Stem hardware, stud type crank nuts, suspension hardware
8mm x 0.75mm    Chainring bolt
9mm x 1mm   Front hubs, quick release, Asian manufacturer
9mm x 26 tpi    Front hubs, Campagnolo®
3/8 inch x 24 tpi   Some solid axle bike, including coaster brake
3/8 inch x 26 tpi   Solid rear axle
10mm x 1mm  Most quick release rear axles, derailleur mounting bolts, brake lever adjusting barrels
10mm x 26 tpi   Rear axle, quick release, Campganolo®
12mm x 1mm  Some spline crankset bolts
1/2 inch x 20 tpi   Pedal threads, one-piece cranks
9/16 inch x 20 tpi  Pedal threads- common three piece cranks
14mm x 1mm  Oversized frestyle axles
15mm x 1mm tpi  Crank bolt, Octalink® and ISIS Drive®
1-inch x 24 tpi     Threaded headsets, one-inch standard
1-1/8 inch x 26 tpi     Thread headset, 1-1/8 inch standard
1-1/4 inch x 26 tpi     Thread headset, 1-1/4 inch standard
1.37 inch x 24 tpi  Bottom brackets, ISO/English/BSC, and threaded freewheel hubs
1-3/8 inch x 26 tpi     Bottom brackets, older “Raleigh” three speeds
36mm x 24 tpi   “Italian” threaded bottom brackets
  • Welcome to SE Bicycles, and than you for your contribution. What part of this wall of text is actually answering the question? Please browse the tour to learn how SE is different. Your answer looks factually right, but is mostly tangential to the question. – Criggie Mar 28 '17 at 20:02

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