Allen key

Anyone who maintains their own bicycle quickly realizes that almost everything on the bike is held together with Allen screws – but not all in a single size. The number of screws needed depends on the type of bicycle and the components used, but a few sizes appear so frequently that you should definitely have them in your set.

The 4 mm size It's the most common type of bolt on a bicycle. You'll find it on brake lever clamps, derailleur mounts, smaller stem bolts, and many accessory brackets. Hardly any maintenance job can be done without a size 4 bolt.

5 mm This is the second most important size. Seatpost clamps, bottle cage bolts, pedal cleats, and numerous handlebar clamps are tightened with this size. Chainring bolts on older cranks are also often 5 mm.

6 mm It is primarily used on stems – both on the handlebar clamp and on the fork stem clamp – as well as on crank bolts in square taper bottom bracket systems and some seatpost clamps.

8 mm You'll need this for larger crank bolts (such as those found on older Hollowtech or Square Taper cranks) and for some pedal axles that are installed using an Allen key. This size also occasionally appears on bottom bracket caps that don't require a special tool.

Less common, but still useful in the household: 2.5 mm for shifter cover screws or small computer mountings, 3 mm for dropout screws and lighter accessories as well as 10 mm for particularly stressed connections such as hub axle nuts or certain e-bike motor mounts.

Practical tip: Instead of individual wrenches, a folding set with L-profile wrenches (2–10 mm) is recommended, supplemented by a bit set with a T-handle or a torque wrench for safety-critical screw connections. This way you have everything compactly together and can use the shorter arm when space is limited, or the longer arm for more leverage on stuck screws.

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Those who maintain modern bicycles will increasingly encounter the Torx profile alongside the classic Allen key (hex key) – a star-shaped drive with six evenly rounded flanks. Both profiles are located in the screw head, both are actuated from the inside, and at first glance, the screw heads look similar. The difference lies in the geometry and the resulting properties.

The Hex socket profile (Allen key) It transmits force across six straight edges. This works well with moderate torques, but has a weakness: if the wrench doesn't fit perfectly or is slightly worn, it begins to "round" the edges – the tool slips out of the screw head. This happens particularly quickly with small screws (2.5 to 4 mm) because the overall contact area is small.

The Torx profile (Manufacturer designations: T-Star, Six-Lobe) distributes the force across curved flanks that pull into the tool instead of pushing out when tightened. This is called "cam-out resistance": The wrench stays more securely in the screw, even when a slightly higher torque is applied. That's why component manufacturers like Shimano, SRAM, and many frame builders now prefer Torx for safety-critical connections – for example, on brake rotors, disc adapters, derailleur hanger bolts, and handlebar faceplate screws.

For you as a cyclist, this means you need both. Allen keys (2–10 mm) remain indispensable for the vast majority of screws. In addition, you'll need a Torx set, at least in the following sizes: T10, T20, T25 and T30, because brake rotor screws are almost always T25 and derailleur mounts on SRAM often require T25 or T30.

Never confuse the profiles and never use Allen keys in Torx screws or vice versa: The flanks are shaped differently, and an "almost fitting" key will damage the screw head within a few turns. For maximum safety, buy a combination set that combines both profiles in a single folding unit.

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A good Allen key made of chrome vanadium steel will last for many years if used properly. But it's not a consumable item that you keep an eye on – and that's precisely where the danger lies, because a worn key often damages screw heads faster than you realize.

The most important warning signal is Edge roundingIf the six edges of the wrench are no longer sharp and angular, but appear slightly rounded or shiny and flattened, it no longer transmits force evenly. It may still be seated in the screw, but it will spin out when tightened – especially under load. Check this by looking at the cross-section: A new wrench has razor-sharp, defined edges; a worn one looks slightly faceted or polished to a shine.

A second sign is Torsional distortionVery inexpensive keys, or those that have been overloaded, can twist slightly. You can see this if you place the key on a flat surface and it tilts slightly, or if the short and long shanks are no longer at a perfect right angle.

corrosion Another problem is rust, especially if you store keys in damp or unprotected conditions. Light surface rust can be removed with oil and a rag, but deep pitting corrosion on the edges renders the key unusable – it will then break under load, sometimes abruptly.

When to replace? Immediately if a wrench slips out of a screw head and damages the screw – that's a clear sign it's no longer usable. As a preventative measure, it's recommended to replace wrenches from inexpensive sets every two to three years or exchange them for a high-quality brand-name set (Wera, PB Swiss, Wiha) that will still have precise edges even after years of use. When it comes to safety-critical screws on a bicycle – stem, handlebars, seat post – tool quality is not something to skimp on.