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An M8 screw has a metric thread with a nominal diameter of 8 mm. That number does not tell you its length, head dimensions, material, strength class or thread pitch. For a typical coarse-thread M8 fastener, the pitch is 1.25 mm; fine-pitch M8 threads also exist, so check the full specification and the mating part before choosing one.
What does M8 mean?
In an ISO metric thread designation, “M” identifies a metric thread and “8” is its nominal major diameter, measured across the thread crests. The designation refers to the thread, not the head: an M8 screw does not necessarily have an 8 mm-wide head or a particular drive style. Basic dimensions for general-purpose ISO metric screw threads are specified in ISO 724:2023.
What pitch is an M8 screw?
Pitch is the distance from one thread crest to the next. M8×1.25 is the commonly used coarse-pitch version, while M8×1 is a fine-pitch option. Because “M8” alone does not specify pitch, it may not be enough to identify the correct fastener or mating thread. ISO 8765:2022 includes fine-pitch hexagon-head bolts from M8×1 through M64×4; a dimensional guide also identifies 1.25 mm as the usual M8 coarse pitch.
When replacing a fastener, match its pitch to the threaded hole or nut. A screw with the right nominal diameter but the wrong pitch will not properly fit the mating thread. If the existing fastener’s pitch is unknown, identify it before ordering; a metric thread-pitch gauge can help compare the thread spacing.
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What information does “M8” leave out?
Thread designation, product geometry and mechanical properties are separate parts of a fastener specification. Check each of the following rather than assuming M8 determines them:
- Pitch and thread fit: Confirm coarse or fine pitch and compatibility with the mating thread.
- Length: Specify the required length and use the measurement convention for the selected head style.
- Head form and drive: Hexagon, countersunk and other head styles have different dimensions and ways to turn them. M8 does not specify a wrench size or drive.
- Material and finish: Choose a material or coating appropriate to the environment. The M8 designation alone does not establish corrosion resistance.
- Property class: Select the required mechanical properties for the application; M8 by itself does not establish strength.
- Joined material and application: The component or substrate being fastened affects what fastener is appropriate.
How to choose an M8 screw or bolt
- Identify the mating thread. Confirm that it is metric M8 and determine whether its pitch is 1.25 mm, 1 mm or another specified pitch.
- Choose the product form. Decide whether you need a screw or bolt and select the head style and drive to suit access and the component.
- Set the length. Choose the required length for the joint, accounting for the head style and the way length is measured for that product.
- Choose material, finish and property class. Base these on the environment and the application’s requirements, not on the M8 label alone.
- Check the complete specification against the product standard and mating part. For example, ISO 4017:2022 covers steel and stainless-steel hexagon-head screws with coarse-pitch metric threads from M1.6 to M64, in product grades A and B. Its scope is specific to those products; it does not mean every M8 fastener has the same geometry or properties.
When do M8 fine-pitch bolts matter?
Fine-pitch M8 bolts are a defined option, not a different diameter. ISO 8765:2022 covers fine-pitch hexagon-head bolts from M8×1 through M64×4, in product grades A and B. If a component specifies M8×1, do not substitute an M8×1.25 fastener simply because both are nominally M8; the pitch must match.
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Does an M8 size tell you how much load it can take?
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Quick M8 selection checklist
- Is the thread metric M8, and does its pitch match the mating part?
- Is the required length clear for the chosen head style?
- Are the head form and drive correct for the component and access?
- Are material, finish and property class suitable for the environment and application?
- Have you checked the fastener’s product standard and the needs of the joined material?
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