Bicycles Asked by jimchristie on February 13, 2021
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.
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.)
Answered by Goodbye Stack Exchange on February 13, 2021
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.
Answered by Chris H on February 13, 2021
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