How To: Properly Measuring and Classifying bolts

Here is a guide on how to measure your hardware and we will also explain to you the reasoning behind the technical language of our products.

A common error when talking about fastener sizing is to refer to the tool used to install the bolt. A bolt that uses a 5mm hex tool interface is not necessarily a M5 bolt. On bicycles most common M5 bolts use a 4mm tool and most M6 bolts use a 5mm tool.

The hardware that we sell is all based on the Metric measuring system for fasteners. We offer hardware in M10, M8, M6, M5, M4 and M3.

We also sell standard hardware with several head styles: Capped, Tapered, Countersunk, Button Head and Flat mount. 


Here is the breakdown on the name of a bolt:


M- Indicates this bolt uses metric threads

5- Refers to the nominal diameter of the bolt shaft in millimeters

X- Separates the Metric size of the threads and how long the bolt is.

18- This means the threaded part of the bolt is 18mm long.


Please note that Capped and Countersunk fasteners are measured differently, we will get into that shortly.


Tool needed to accurately measure a bolt: Digital Caliper set to millimeters 

So let’s begin measuring bolts!

Here is an example of measuring a bolt to determine what metric size it is.

As seen in the above photo, this bolt diameter is 5.93mm. We round this number up to 6mm. This tells us it is an M6 bolt. 

If this bolt used a 3mm hex tool to install it on the bike, it is still a M6 bolt.

This bolt has a 5mm hex interface and is a M6 bolt.

So here is the information we know: This is a M6, now we need to find the second part of the measurement.

When measuring Capped, Tapered, Flat or Button head hardware we measure from the base of the head.

This bolt is 40mm long.

Now we know that this bolt is a M6X40mm and it has a tapered head.


Here are some examples of the fastener heads we offer:

These are our Rotor and Water bottle cage bolts. These both use Button Heads. These are both M5 and use T25 and 3mm tool interfaces.


Here is an example of M6 and M5 bolts with Tapered Heads. 


This photo shows a Countersunk (Left) and a Capped Head (Right)


This is an example of a Capped Head that is also Knurled.

Knurling refers to the texture that is added to the bolt head during manufacturing. 


Earlier I mentioned that Countersunk bolts are measured differently:

Both of these bolts are M6X35mm. 

Yes the total length of the bolts are different but they are technically the same size and just have different heads.

Countersunk head is measured from the top of the head.

Capped, Tapered  and Button Head bolt are measured from the base of the head.


Next Topic: Washers

At Better Bolts we offer 2 different size washers.

M6 and M5 Washers

M6 Washers work with any length M6 bolt

M5 Washers work with any length M5 bolt

Please note that you would never use a washer with a Countersunk bolt.

When measuring washers, you need to find the diameter of the inside surface.


If you have any questions regarding the hardware that you need please do not hesitate to reach out via our contact form Here.

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