Thru Axle Sizes Explained: How to Measure Your MTB Thru Axle & Find the Right Replacement
Confused by MTB thru axle sizes? Learn how to measure axle length, diameter, thread pitch and thread length to find the correct replacement for your bike.
If you need to replace a mountain bike thru axle, getting the correct size is essential. Two axles can look almost identical but have different lengths, thread pitches or thread lengths — meaning the wrong one simply won’t fit your bike.
The safest way to find the correct replacement is to remove your existing thru axle and check its diameter, shaft length, thread pitch and thread length.
In this guide, we’ll explain how to measure an MTB thru axle correctly, what common sizes such as 12×142, 12×148 Boost and 15×110 Boost actually mean, and how to match your measurements to the right replacement axle.
Important: A size such as 12×148 describes the hub standard — it does not mean the thru axle itself is 148mm long.
What Is a Thru Axle?
A thru axle is the removable axle that secures your wheel to the bike’s fork or frame.
Unlike a traditional quick-release (QR) skewer, a thru axle passes through one side of the fork or frame, through the wheel hub and then threads securely into the opposite side or a separate threaded fitting.
Thru axles provide a strong, stiff and secure connection between the wheel and bike, which is why they are now used on the vast majority of modern mountain bikes.
What Thru Axle Size Do I Need?
When replacing a thru axle, don’t rely solely on terms such as Boost, 12×148 or 15×110.
The safest approach is to remove your existing axle and check the following:
1. Axle diameter
2. Axle length
3. Thread pitch
4. Thread length
You should also check that the axle head and fitting arrangement are compatible with your bike.
Getting all of these measurements right is important — an axle with the correct diameter and length can still be incompatible if it has the wrong thread.
How to Measure a MTB Thru Axle
1. Check the Axle Diameter
Modern mountain bikes predominantly use two thru axle diameters:
· 15mm — commonly used on MTB front forks
· 12mm — commonly used for MTB rear wheels
You may also encounter:
· 20mm — used on some downhill, freeride and heavy-duty forks
The diameter may be printed or engraved on the original axle. If not, it can be measured accurately using a vernier or digital caliper.
2. Measure the Axle Length
This is where thru axle sizing can become confusing.
The physical length of the axle is not necessarily the same as the hub spacing.
For example:
12×148 Boost does not mean the replacement axle is 148mm long.
The 148mm measurement refers to the width of the rear hub standard. The actual thru axle has to pass through the frame as well as the hub, so its physical length will normally be considerably longer.
Don’t Measure Your Hub and Assume That’s Your Axle Length
If your bike has a 12×148 Boost rear hub, don’t simply search for a 148mm-long axle.
Instead, remove your existing axle and measure the shaft.
Measure from the inside face of the axle head to the end of the axle, rather than including the external head itself.
When comparing your measurement with a replacement axle, always check how the seller defines axle or insertable length.
At EJ’s Bikes, our axle listings provide detailed measurements to make comparing your existing axle with its replacement as straightforward as possible.
3. Check the Thread Pitch
Thread pitch is the distance between each thread.
This is extremely important because two axles can have the same diameter and overall length but completely different threads.
Common 12mm MTB thru axle thread pitches include:
· M12 × 1.0
· M12 × 1.5
· M12 × 1.75
For example, M12 × 1.5 means the axle has a 12mm diameter with 1.5mm between each thread.
How Do I Find My Thru Axle Thread Pitch?
The easiest methods are:
· Check the markings on your existing axle
· Check your bike or frame manufacturer’s specification
· Measure it with a thread pitch gauge
· Compare it carefully with the specifications of a known axle
Don’t assume the thread pitch based purely on the bike, frame or component brand. Manufacturers can use different thread pitches across models and generations.
Never force an axle into a thread that doesn’t turn smoothly. Using the wrong thread pitch can damage the frame or threaded insert.
4. Measure the Thread Length
Thread length is another measurement that can easily be overlooked.
Measure the length of the threaded section from where the threads begin to the end of the axle.
For example, two axles could both be:
12mm diameter × 169mm long × M12 × 1.5
but have different thread lengths.
For the best fit, compare the thread length of your original axle with the replacement as well as the overall dimensions.
Common MTB Thru Axle and Hub Standards
You will frequently see dimensions such as 15×110 and 12×148 when looking for replacement MTB axles.
These generally describe the hub standard, rather than the physical length of the replacement thru axle.
|
Position |
Common Standard |
Hub Spacing |
Typical Use |
|
Front |
15×100 |
100mm |
Non-Boost MTB |
|
Front |
15×110 |
110mm |
Boost MTB |
|
Front |
20×110 |
110mm |
Downhill / heavy-duty MTB |
|
Rear |
12×142 |
142mm |
Non-Boost MTB |
|
Rear |
12×148 |
148mm |
Boost MTB |
|
Rear |
12×157 |
157mm |
Super Boost / downhill |
Remember: these dimensions describe the wheel/hub standard. They don’t necessarily tell you the physical length or thread pitch of the axle you need to buy.
What Does Boost Mean?
Boost is a wider MTB hub standard designed to increase wheel stiffness and provide additional clearance around the tyre, frame and drivetrain.
The two most common Boost standards are:
Front: 15×110mm Boost
Rear: 12×148mm Boost
If your bike uses a 12×148 Boost rear hub, you still need to determine the actual axle length and thread pitch before ordering a replacement.
The same applies to a 15×110 Boost front axle.
How to Read a Thru Axle Size
Once you understand the measurements, thru axle specifications become much easier to interpret.
For example, an axle might be described as:
12mm × 169mm — M12 × 1.5 — 17mm thread length
This tells you:
· 12mm — axle diameter
· 169mm — axle length
· M12 × 1.5 — thread diameter and pitch
· 17mm — length of the threaded section
Compare each of these measurements with your existing axle before ordering.
When Should You Replace a Thru Axle?
Thru axles are strong components, but they can become damaged or worn.
Consider replacing your axle if you notice:
· Damaged or stripped threads
· A bent or deformed axle
· Damage following a crash or heavy impact
· Significant corrosion or pitting
· A damaged axle head or hex fitting
· An axle that no longer tightens correctly
If an axle is damaged or no longer secures the wheel correctly, don’t continue riding until the problem has been resolved.
Finding the Right Replacement Thru Axle
The simplest way to find the correct replacement is to remove your existing axle and compare its dimensions directly with the replacement.
Check:
Diameter → Length → Thread Pitch → Thread Length
Don’t choose an axle purely because it’s advertised as Boost, 12×148, 12×142 or 15×110.
Once you know your measurements, you can compare them directly with the detailed dimensions shown on EJ’s Bikes thru axle listings.
If you’re still unsure which axle you need, send us the measurements or details of your existing axle and we’ll help you find the right fit.
MTB Thru Axle FAQs
Are All 12×148 Thru Axles the Same?
No.
12×148 describes a common Boost rear hub standard. Bikes using 12×148 hubs can require thru axles with different physical lengths, thread pitches, thread lengths and fitting arrangements.
Always check the specifications of your existing axle.
Is a 12×148 Thru Axle 148mm Long?
Not necessarily — and usually not.
The 148mm refers to the hub spacing, not the total physical length of the thru axle.
The axle also passes through the frame dropouts, so the actual axle is normally longer than the hub spacing.
Are All 15×110 Boost Thru Axles the Same?
No.
Although 15×110 describes the front Boost hub standard, different forks can use different axle designs and specifications.
Check your original axle and fork specifications before ordering.
How Do I Know My Thru Axle Thread Pitch?
First check the original axle, as the size may be printed or engraved on it.
If it isn’t marked, check the bike or frame manufacturer’s specifications or use a thread pitch gauge.
Common rear MTB axle pitches include M12 × 1.0, M12 × 1.5 and M12 × 1.75, but you should always verify your particular bike.
What’s the Difference Between M12 × 1.0, M12 × 1.5 and M12 × 1.75?
All three have a nominal thread diameter of 12mm.
The second number describes the distance between the threads:
· M12 × 1.0 = 1.0mm thread pitch
· M12 × 1.5 = 1.5mm thread pitch
· M12 × 1.75 = 1.75mm thread pitch
They are not interchangeable.
Can I Use a Longer Thru Axle?
You should use an axle that matches the specifications required by your frame or fork.
An axle that is too short may not provide sufficient thread engagement, while an axle that is too long may protrude excessively or fail to secure the wheel correctly.
Always compare the replacement with the dimensions and specifications of the original axle.
What’s the Easiest Way to Find My Thru Axle Size?
If you still have the original axle, remove it and check:
1. Diameter
2. Axle length
3. Thread pitch
4. Thread length
Then compare those measurements with the replacement axle specification.
This is generally much more reliable than searching for an axle based solely on your hub size or the make of your bike.
Shop MTB Thru Axles at EJ’s Bikes
EJ’s Bikes stocks a range of lightweight, high-strength MTB thru axles in popular sizes and thread pitches for modern mountain bikes.
Every axle listing includes detailed dimensions to help you compare it with your existing axle and choose the correct replacement.
Not sure which one you need? Get in touch with EJ’s Bikes and we’ll help you identify the right axle for your bike.