What size is a bike Bolt?
There isn’t a single size that covers all bicycle bolts. In practice, bikes use a range of metric bolt sizes—most commonly M4, M5, M6 and M8—with through-axles often bigger (12 mm or 15 mm) and rotor bolts typically M5 x 0.8.
Bolt size depends on the component and mounting context. This article explains the common sizes by component, how to identify them on a bike, and practical tips for maintenance and replacement.
Common bolt sizes by component
Below is a snapshot of typical sizes used across the main components. Actual sizes can vary by model, so Always verify with the product manual or manufacturer specs.
- Seat post binder bolts: typically M5 or M6, with 4–6 mm hex heads; lengths usually 8–20 mm depending on the seat clamp design.
- Stem clamp bolts (handlebars): commonly M6, with 5–6 mm hex heads; lengths around 10–25 mm.
- Brake caliper mounting bolts (disc brakes): often M6 or M8; common hex sizes 4–5 mm; lengths vary by mounting style.
- Disc rotor bolts: usually M5 x 0.8 thread; commonly require a 2.5–3.0 mm hex key; length around 6–8 mm.
- Crank bolts: many square-taper and some ISIS cranks use roughly M8 bolts (8 mm diameter) that thread into the spindle; some external BB setups use longer bolts (M8 or M12) depending on design; pitch commonly 1.0 mm.
- Bottom bracket bolts (crank arm bolts on external BB systems): often M8 or M12, depending on crank design; torque per manufacturer guidelines is essential.
- Thru-axles: front typically 12x1.5 mm or 12x1.0 mm; rear commonly 12x1.75 mm or 12x1.0 mm, with some frames using 15x1.0 mm variants.
Concluding note: sizes vary widely by component and frame. Always confirm against the bike’s manual or the exact bolt you are replacing to ensure compatibility and safety.
How to identify bolt size on your bike
Identifying the correct bolt size involves checking specifications and, when needed, measuring directly. The process helps avoid cross-threading or under/over-tightening critical parts.
- Check the manufacturer’s specifications or service manual for the exact bolt size and torque settings for each component.
- Measure the bolt’s diameter with calipers or a bolt gauge, and determine the thread pitch per millimeter (TPI/pitch) using a thread gauge or by comparing with known standards.
- Measure the bolt length (excluding the head) to ensure replacement matches the engagement depth.
- Identify the head type (hex, socket, button, etc.) to select the correct driver or Allen key size and to ensure proper clearance with surrounding parts.
- For replacement, choose bolts made of appropriate material (steel vs stainless), with the correct coating, and torque to the manufacturer’s specification using a torque wrench when possible.
Note: Through-axles and some crank systems have frame-specific standards. If in doubt, bring a sample bolt to a reputable bike shop or consult the frame and component manuals to ensure correct fitment.
In short, selecting the right bolt size is component-specific. The most reliable path is to reference the bike’s documentation or to measure the existing fasteners and match the pitch, diameter, length, and head style precisely.
Maintenance, replacement, and buying tips
Materials and quality
Most bike bolts are steel, stainless steel, or alloy. Stainless offers better corrosion resistance for outdoor riding. For safety-critical parts such as brakes and cranks, use bolts that meet or exceed the manufacturer’s strength grade and follow torque specifications. Avoid reusing damaged or stripped bolts.
Where to buy
Spare parts are available through bike shops, manufacturer catalogs, and reputable online retailers. When ordering, specify the exact size (diameter x pitch), length, and head type. If replacing a through-axle, confirm the exact frame standard (for example, 12x1.5 or 12x1.0) as these aren’t interchangeable between frames.
Summary
Bike bolts come in a range of sizes tailored to each component. Common metric diameters include M4, M5, M6 and M8, with through-axles often at 12 mm or 15 mm and rotor bolts typically M5 x 0.8. Always verify exact sizes via manufacturer specifications or direct measurement, and torque to the recommended settings to ensure safety and performance.
What size is the bolt on a bike wheel?
Fasteners: One 5mm bolt, behind the top of fork below down tube, and one on the rear brake bridge above the wheel. Some brakes use a T25 bolt head.
What size bolts are on bikes?
Most bolts that are used to mount accessories/minor parts to bicycles are sized M5 but some manufacturers also use M6, M7, or M8. The thread pitch (i.e. distance between threads) on an M5 bolt is around 0.8mm. The majority of bolts found on bikes can be tightened using either an Allen key/hex wrench or Torx wrench.
Are bike bolts metric or standard?
metric
Modern bicycle bolts are mostly metric. They may however have non standard heads, use non standard hex keys for their diameter and have fine rather than regular threads. Bolts also vary in material and strength, so you can buy low and high tensile strength bolts and also plain steel, plated, treated or stainless steel.
What size are bike stem bolts?
Bike stem bolt sizes are typically M5 or M8 for mountain/road bikes, but can vary depending on the stem and bike type, so it's best to check the stem manufacturer's specifications. The most common bolt is M5 x 18mm for the faceplate and M8 x 1.25mm for the steerer clamp bolt, though some manufacturers use different dimensions like M5x20mm or other sizes. It is crucial to verify the specific bolt size and torque setting for your bike.
This video demonstrates how to change your bike stem, including removing and tightening stem bolts: 59sThe Pro's ClosetYouTube · Sep 28, 2022
Common stem bolt sizes
- For the faceplate (handlebar clamp): These bolts are often M5, such as M5 x 18mm or M5 x 20mm.
- For the steerer clamp (connecting stem to fork): These are frequently M8, with a common size being M8 x 1.25mm.
- For the top cap: The top cap bolt is often a 4mm or 5mm hex, and may have a different size specification than the other stem bolts.
How to confirm the correct size
- Check the manufacturer's specifications: Look for the torque specification (in Nm) or bolt size stamped on the back of the stem.
- Consult your bike's manual: Refer to the owner's manual for your bike model for specific requirements.
- Measure the existing bolt: If you are replacing a bolt, you can measure the diameter and length of the old one to find its specifications.
- Use a bolt gauge: A simple bolt gauge can help identify the correct size by pushing the bolt through holes of different sizes until you find the one it fits.
