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There's one size of bolt that's used all over modern bikes: water bottle mounts and the braze-ons for racks leap to mind. Whenever I need a few, I take one to the hardware store and sift through the boxes of bolts until I find one that matches. I foolishly never write the size down.

What size are they? I'm only concerned with the width and the threads, not the heads or the length.

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Unfortunately, they don't sell 35mm film anymore, since I've always kept a film can of assorted screws in my toolkit. –  Daniel R Hicks Sep 26 '13 at 3:22
    
You can get 35mm film all over the place Daniel. It's still used by a lot of people. Filing that, you can find plenty of film canisters for sale all over the internet. –  alex Sep 26 '13 at 5:27
    
@DanielRHicks I find that empty medicine bottles work just as well. I use both on my computer bench. New ones even have easy threads (ie, a none child safe thread) on the otherside of the cap. Baby food jars work just as well. –  BPugh Sep 26 '13 at 12:35

2 Answers 2

The most common sizes of allen screws for bikes are 4, 5, and 6mm.

There's even a common bike tool that will fit in all three. In my experience, most racks use 5mm screws, but some do use 4 or 6mm screws.

Allen screws are generally used because flathead screws take longer to turn as the head slips easily, and phillips head screws strip easily. (I've heard that allen screws also work well with torque wrenches, but I can not verify this.)

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When you're buying screws from the hardware store, knowing only the diameter isn't all the information you need. You need the threading as well, as the OP said. –  Carey Gregory Sep 26 '13 at 5:16
    
For "used all over modern bikes", they will be standard metric threads, so just "M5" is enough. If it's an older bike, you have to worry that 2BA is close enough to M5 that you can't tell them apart at a glance. –  armb Sep 26 '13 at 10:24
    
en.wikipedia.org/wiki/ISO_metric_screw_thread#Designation 'If the pitch is the normally used "coarse" pitch listed in ISO 261 or ISO 262, it can be omitted' (As Chris H said, basically.) –  armb Sep 26 '13 at 14:01
    
@CareyGregory - Hardware stores by me only stock one thread size, so I've never had to worry about that. Nevertheless, have edited my answer. –  Neil Fein Sep 30 '13 at 15:06

Strictly speaking the answer should be M4, M5 or M6. This specifies the threading as well* (M4x0.7, M5x0.8 and M6x1 is implied, where the thread is given in mm/turn). The hex key for turning them will also have a size in mm, always smaller than the diameter, by how much depends on the head shape.

I would say M5 are the most common, based on a very small sample size.

*There are technically fine and extra fine metric threads, but you won't find them on a bike or in a hardware store.

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I've never had trouble buying stainless metric screws at a hardware store. They're in the drawer marked "stainless steel metric cap screws". The key is to take a sample with you, to match the threads. –  Daniel R Hicks Sep 26 '13 at 11:21
    
@DanielRHicks I'm in the UK so it's easy for me too, however not knowing where the OP is looking for his/her screws I thought a little more detail would be in order. Personally I buy SS screws on ebay - my local good hardware shop isn't so good for stainless, or very convenient, and I rarely need them "right now" (probably because I bought 10 last time I needed 2 so have a stock). –  Chris H Sep 26 '13 at 11:24
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My local hardware store almost certainly does have fine thread screws too. But they won't be in the same drawer as the normal coarse thread screws. And surely the point of the question is that if you know what size you want, you don't need to take a sample. –  armb Sep 26 '13 at 14:05
    
@armb I want your local hardware store near me if they have metric fine machine screws! For a generic question, the answer can only be generic, so I think the best anyone can say is along the lines of what I said. However opinions and experience may vary on which of the common sizes is most likely. –  Chris H Sep 26 '13 at 14:10
    
Most US hardware stores are equipped with small cabinets with several drawers that contain special screws and other fasteners. One of the drawers will almost certainly be SS metric cap screws. They're not in the big bins or in bags hanging on the display rack. –  Daniel R Hicks Sep 26 '13 at 15:08

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