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I am trying to install a front wheel in a bike - the axle of the wheel is threaded, but I have 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 of a 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 did fit 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

3 Answers 3

up vote 1 down vote accepted

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

There is a spiffy table here: http://www.sheldonbrown.com/harris/axles.html that may clear it up for you.

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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

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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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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