What size are the nuts holding the front wheel?
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.
For those coming here looking for a wrench size for the bolt, it's probably 15 mm.
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.
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.
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
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.
Edit: while it's probably too much to include the whole table, the ø8mm sizes are reproduced below.
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
FWIW 8x1.25 is the (coarse) default for M8, but 8x1 and even 8x0.75 are allowed as the "fine" pitches by ISO 262.
So, 8x1 isn't exclusive to bike use or non-standard, it's just less common.
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.
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.
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.
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
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