Summary of hex wrench sizes to prepare for road bike maintenance.

Posted at: May. 16, 2022

For those who will be servicing their road bikes, we have compiled a list of hex wrench sizes that are commonly used for road bike maintenance and that you should have available.

Table of contents

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.

Parts	Size of Allen Wrench
Seat post clamp	5mm
Brake wire clamp	5mm, 4mm
Brake mounting bolts	5mm, 4mm
Chainring bolt	5mm
FD wire bolt	5mm, 4mm
RD wire bolt	5mm, 4mm
Wheel hub	5mm, 10mm

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.

Parts	Size of Allen Wrench
Brake shoe bolts	4mm, 5mm
bottle cage bolts	4mm
Brake wire clamps	5mm, 4mm
FD wire bolt	5mm, 4mm
RD wire bolt	5mm, 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.

Parts	Size of Allen Wrench
Pedals 6mm, 8mm
STI lever mounting bolts	6mm, 8mm

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.

Parts	Size of Allen Wrench
Pedals	6mm, 8mm
Shaft cover	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.

Mechanics New Posts

Latex Butyl? Different tire tube materials and how to choose the right one.

There are two types of bicycle tire tubes, latex tubing and butyl tubing. The following is a summary of the differences between the two and which one you should choose.
Guide to Choosing Binding Pedals for Road Bikes

Binding pedals allow you to ride faster and more efficiently on your road bike. In this article, we will thoroughly explain what to look for when choosing binding pedals.
Summary of binding pedal cleat types and their respective characteristics

Cleats are the parts that connect binding pedals to shoes. Cleats are only one part, but each manufacturer has its own characteristics and differences. They are actually important parts that are linked to usability and their merits and demerits.
Advantages and disadvantages of putting sealant in road bike and MTB tires and how to choose

This section summarizes what sealants are for road bikes and MTBs, their advantages and disadvantages, and how to select sealants.
Types of valve extenders and how to select one

A valve extender, which extends the valve, is an essential item for operating deep-rim wheels on road bikes and other vehicles. The following is a summary of the different types of valve extenders, their differences, and how to select one.
Summary of brake shoes for carbon rim and wheel

When using rim brakes on carbon rim/wheels, it is essential to use special brake shoes to reduce braking force and damage to the rim. In this issue, we introduce a collection of brake shoes for carbon rim wheels.
Advantages and disadvantages of front single

The "front single" without front shifting is being increasingly adopted in MTB and gravel road bikes. This section summarizes the advantages and disadvantages of the front single.
Road Bike Stem Standards and How to Choose

The stem is an unassuming part, but actually has a significant impact on ride comfort and speed. In this issue, we summarize road bike stem standards and how to select one.
Advantages and disadvantages of tubeless and tubeless ready tires

Tubeless and tubeless ready tires are increasingly being adopted in the latest wheels. The following is a summary of the differences, advantages and disadvantages of each, and points to keep in mind when choosing.
Advantages and disadvantages of using CO2 cylinders

The CO2 cylinder is the preferred puncture repair inflator for expert road bike users. This section summarizes the advantages and disadvantages of using CO2 cylinders for inflating, and the differences from portable pumps.
Advantages and disadvantages of tubular tires

Tubular tires are the choice of users who are concerned about performance. In this issue, we summarize the advantages and disadvantages of tubular tires and what kind of users they match.
How to customize a road bike seat post and the effects you can get

Seat post customization has become a standard because it is easy and simple to do, even for beginners of road biking.
How do you determine the height of the handlebars on a road bike?

The "handlebar height" of a road bike has a significant impact on speed as well as ease of riding. This section summarizes what happens when you change the height and how to determine the right height.
Shimano road disc brake component compatibility summary

Shimano disc brake components for road use are difficult to select if you don't know about them. We have compiled a list of parts model numbers and their incompatibility, which can be difficult to understand.
What is the Q Factor? Measurement methods, how to determine appropriate values, and how to adjust

Q-factor is an important factor for improving pedaling efficiency. we summarize what Q-factor is, how to measure it, how to determine the proper value, and how to adjust it.
LOOK's Grades and Types of Binding Pedals for Road Use

LOOK is a highly regarded manufacturer of complete vehicles, and in this article, we'll take a look at the grades and types of LOOK's popular binding pedals for road use, and the differences between each model.
Summary of FSA cranks, grades and ratings

FSA cranks are often chosen as standard equipment for complete bikes. Here is a summary of their grades and why they are often rated low.
Choosing the right tires for a rainy road bike ride

Road bikes are often used for riding in the rain. In this article, we summarize tire selection for riding in the rain on a road bike.
What changes when you upgrade your Shimano rear derailleur?

One of the most modest component upgrades is the rear derailleur. Here is a summary of what changes when you upgrade your rear derailleur.
How to lighten heavy mechanical brakes.

The weight of the brakes not only affects the braking power, but also creates a significant impact on fatigue during the ride. In this article, we'll take a look at how mechanical brakes work and how you can lighten the weight of heavy brakes.