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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
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
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
up vote 1 down vote accepted

There's a wide variety of axle diameters and threading.

There is a spiffy table here: that may clear it up for you.

Here is another list of common bicycle thread sizes from Park Tools.

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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
the link appears to be broken – RoboKaren Aug 2 '15 at 4:36
Answers should include answers, not just links. – jqning Nov 8 '15 at 4:21

Since the accepted answer consists only of a broken link (which is why link-only answers are so awful) and this question is ranked high by google, here's a late answer.

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

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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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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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