How to Measure Your Motorcycle Bolts
How to Measure Your
Motorcycle Bolts
Four measurements, one correct bolt. Here's how to get it right every time.
Getting the right replacement bolt for your motorcycle comes down to four simple measurements: thread diameter, thread pitch, bolt length and thread length. Get these right and you'll find an exact match every time. This guide walks you through each one.
Metric or Imperial?
The vast majority of modern motorcycles use metric fasteners. Unless your bike was manufactured in the United States, you can safely assume it's metric — and metric is what you'll find across our entire range.
Metric and imperial threads are not interchangeable. Never try to fit a metric bolt into an imperial thread or vice versa. The threads may feel like they're engaging, but they'll cross-thread and damage both the bolt and the component. If you're unsure which standard your bike uses, check your service manual or get in touch and we'll help you confirm.
Bolt Anatomy
This diagram shows the four key dimensions you'll need to identify when measuring a motorcycle bolt:
Bolt dimensions reference — all AMD fasteners use metric measurements (millimetres)
Measuring Your Bolt
Thread Diameter
Measure across the widest point of the threaded section — from the outside of the thread on one side to the outside of the thread directly opposite. This is the major diameter, and it determines the "M" size of the bolt.
A bolt that measures 10mm across the threads is an M10. One that measures 8mm is an M8. Common motorcycle sizes are M5, M6, M8, M10 and M12.
Example: 10mm across = M10
Thread Pitch
The thread pitch is the distance from the crest of one thread to the crest of the next, measured along the length of the bolt. This is the measurement most people get wrong, and it's the one that matters most for getting a proper fit.
A pitch of 1.25mm means there's 1.25 millimetres between each thread crest. The easiest way to measure this accurately is with a thread pitch gauge — a set of small metal blades that you hold against the thread until one matches perfectly.
Example: 1.25mm between crests = 1.25 pitch
Bolt Length
Measure from the underside of the bolt head to the very tip of the bolt. The head itself is not included in the length — only the portion that sits inside the component.
This is measured in millimetres. A bolt that measures 55mm from under the head to the tip is a 55mm bolt.
Example: 55mm from under head to tip = 55mm length
Thread Length
This is the length of the threaded portion of the bolt, measured from where the threads begin to the tip. Not all bolts are threaded along their full length — many have a smooth shank section between the head and the threads.
Thread length matters for fitment. If the original bolt has a specific smooth shank section (common on axle bolts and caliper mounts), you'll need a replacement with the same thread length to ensure it sits correctly in the component.
Example: 20mm of thread on a 55mm bolt = 20mm thread length
Putting It All Together
Once you have your four measurements, they combine into a standard bolt size. Here's how it reads:
The "M" stands for metric. On our website, you'll find bolts listed in this format — so once you've measured yours, matching it to the right product is straightforward.
Common motorcycle thread pitches aren't always obvious. An M10 bolt, for example, could have a 1.25mm or 1.5mm pitch depending on the application. The diameter alone won't tell you everything — always check the pitch before ordering.
Recommended Tools
You can measure bolts with a standard ruler at a push, but for accurate results — especially on thread pitch — the right tools make a real difference:
Digital Vernier Callipers
The most useful single tool for bolt measurement. Gives you accurate readings for diameter, length and thread length in millimetres. A decent pair costs under £15 and will last years.
Thread Pitch Gauge
A set of thin metal blades, each cut to a specific thread pitch. Hold each blade against your bolt's threads until one sits flush — that's your pitch. Inexpensive and eliminates guesswork.
If you've measured your bolt and you're not confident about the size, drop us a message with your bike model and where the bolt sits on the bike. We'll help you find the right one.