Hexagonal wrench sizes commonly used in road bike maintenance
5mm Allen Wrench
The 5mm Allen wrench is the most commonly used tool for road bike maintenance.
It is used for a wide variety of parts such as seat post clamps, saddle attachment lugs, stem bolts, brake wire tightening bolts (some brakes have 4mm, etc.), and chainring bolts. Depending on the parts used on the vehicle, about half of the parts can be serviced with a 5mm Allen key.
Since it is used so often, if you frequently perform maintenance yourself, you should have only a 5 mm hexagonal wrench with a T-type wrench, which will greatly improve your work efficiency.
4mm hexagonal wrench
The next most commonly used type of wrench is the 4mm Allen key.
The bottle cage bolt is 4mm, and the rim brake brake shoe bolt, brake wire retaining bolt, and rear derailleur wire retaining bolt (some models have a 5mm bolt) are also 4mm.
6mm Allen Wrench
A 6mm Allen wrench is used to tighten thicker bolts and shafts.
On road bikes, most pedals that can be tightened with an Allen wrench instead of a pedal wrench are 6mm (8mm for some manufacturers). Also, Shimano STI levers have 6mm bolts that attach to the handlebars.
8mmm hex wrench
The 8mm is used to tighten fairly large bolts; on road bikes, the square BB shaft cover and some pedals are adjusted with an 8mm.
What size hex wrenches should I have if I want to service my road bike?
As we have seen, 4, 5, 6, and 8 mm Allen wrenches are often used in road bike maintenance, so it is a good idea to have those available.
Even simple hexagonal wrenches are often available up to 6mm, and some stores have sets up to 8mm, so you can cover almost all of them. For simple maintenance and installation, simple hex wrenches are usually sufficient.
However, when performing maintenance on deep parts such as STI levers or brake mounting bolts, a hexagonal wrench with a short handle may not be able to reach them, so it is better to buy a special hexagonal wrench for easier maintenance.
For more serious maintenance, for example, disassembly of wheel hubs may require hexagonal wrenches of unusual sizes such as 10mm or 12mm, but such special wrenches can be bought individually when such work becomes necessary. Hexagonal wrenches can be sold individually on the Internet for as little as a few dollars, so it is not cosmetic to buy them in pieces.
Does it have to be an expensive hex wrench?
Phillips and flathead screwdrivers should be used with tools that you can worry about as much as possible, because low-quality tools can lick the screws, but hexagonal wrenches are not highly aggressive to almost all bolts with Phillips or flathead screws, so cheap tools are usually fine.
However, when loosening a bolt that has become stuck, be aware that if the accuracy is not high enough, the bolt may be ruined.
In addition, since expensive tools are not only more accurate but also easier to use, getting a full-fledged hexagonal wrench will speed up the maintenance process if you start doing maintenance frequently. In particular, although they are more expensive, T-type wrenches are recommended because they are really easy to use.