Metric bolts explained for size, thread pitch and property class selection
Metric bolts at a glance
Metric bolts are externally threaded fasteners specified under the ISO metric thread system, with M designations such as M8, M10 or M12. A full callout can identify the nominal diameter, thread pitch, length, product form, strength class, material and coating. In practice, choosing metric bolts is not just a matter of matching the diameter. A reliable specification should confirm the thread, property class, nut compatibility, washer requirements, coating and service environment before the part is purchased or installed. For more related fastener topics, see the Bolts & Fasteners section.
This guide explains the practical meaning behind common markings and standards, with one important limit: final bolt selection for safety-critical, structural, pressure, lifting or vehicle applications should follow the governing drawing, code or engineering calculation.
How metric bolt sizing works
The M in a metric bolt size refers to the ISO metric screw thread form. In a callout such as M10 × 1.5 × 40, M10 is the nominal thread diameter in millimeters, 1.5 is the pitch in millimeters, and 40 is the nominal length in millimeters. For many hex head and socket head bolts, length is measured from under the head to the end of the bolt. For countersunk fasteners, length is commonly measured overall because the head sits flush in the countersink. If a drawing controls the part, follow the drawing convention.
Pitch is the distance from one thread crest to the next. It is not the same as threads per inch, which is used for Unified inch threads. A smaller pitch number means a finer thread. For example, M10 × 1.5 is a coarse-thread M10 form, while M10 × 1.25 and M10 × 1.0 are finer alternatives. When pitch is left out of a casual description, suppliers often assume coarse pitch. That assumption should not be used for maintenance work or procurement when the bolt must mate with an existing nut, tapped hole or insert.
A practical metric bolt description might read: M10 × 1.5 × 40, hex head, property class 8.8, zinc electroplated, ISO 4017. That single line gives the buyer far more information than “M10 bolt” and reduces the chance of receiving the wrong length, thread or strength.
The standards map behind common metric bolts
Metric bolt specifications are usually built from several standards rather than one universal document. A dimensional standard may define the head style and length range, a mechanical standard may define strength, and a coating standard may define surface protection. The table below summarizes commonly referenced documents and the role each one plays in a specification.
| Reference | Main role in a metric bolt specification | Why it matters |
|---|---|---|
| ISO 68-1, ISO 261 and ISO 262 | Metric thread profile, general plan and selected sizes | They support consistent M thread geometry and preferred coarse/fine combinations. |
| ISO 4014:2022 | Hexagon head bolts, product grades A and B | Often relevant where a partially threaded hex bolt form is required. |
| ISO 4017:2022 | Hexagon head screws, product grades A and B, with metric coarse pitch threads | Often relevant where a fully threaded hex head form is required. |
| ISO 898-1:2013 | Mechanical properties of carbon steel and alloy steel bolts, screws and studs | Defines property classes such as 8.8, 10.9 and 12.9 under specified conditions. |
| ISO 3506-1:2020 | Mechanical properties of corrosion-resistant stainless steel bolts, screws and studs | Defines stainless designations such as A2-70 or A4-80, which should not be confused with 8.8 or 10.9. |
| ISO 4042:2022, with Amendment 1:2026 | Electroplated coating systems for fasteners | Addresses zinc and zinc-alloy coating systems and includes hydrogen embrittlement risk considerations. |
| ISO 10684:2004 | Hot dip galvanized coatings for fasteners | Applies to coarse threaded steel fasteners from M8 to M64 and property classes up to and including 10.9 for bolts, screws and studs. |
| ASTM F3125/F3125M | High-strength structural bolts and assemblies in inch and metric dimensions | Relevant to structural steel bolting in ASTM-governed projects, not a general substitute for every metric bolt callout. |
This split between thread, dimensions, strength and coating is why a purchasing line should not rely on size alone. Two M12 × 40 bolts may look similar at a glance but differ in thread pitch, shank length, property class, coating thickness, nut fit and intended service.
How to decode metric bolt property classes
Metric carbon steel and alloy steel bolts usually show a property class on the head, such as 8.8, 10.9 or 12.9. Under the ISO 898-1 system, the first number relates to nominal tensile strength in hundreds of megapascals. The second number represents the ratio between yield strength and tensile strength. As a quick reading, class 8.8 indicates a nominal tensile strength of 800 MPa and a nominal yield relationship of 0.8, giving about 640 MPa as the nominal yield value. Class 10.9 reads as 1000 MPa nominal tensile strength with a 0.9 ratio.
| Property class | Nominal tensile reading | Nominal yield reading | Typical use consideration |
|---|---|---|---|
| 4.6 | 400 MPa | 240 MPa | Low-strength, non-critical fastening where high clamp load is not required. |
| 8.8 | 800 MPa | 640 MPa | A common general engineering class when a stronger carbon steel bolt is needed. |
| 10.9 | 1000 MPa | 900 MPa | Higher-strength assemblies where joint design, coating and installation control become more important. |
| 12.9 | 1200 MPa | 1080 MPa | Very high strength; should not be treated as an automatic upgrade because ductility, corrosion exposure and embrittlement risk must be considered. |
These figures are useful for reading a bolt head, but they are not a complete design method. The standard includes detailed requirements for test conditions, sizes, materials, hardness and other mechanical properties. For procurement, the property class should be paired with the product standard, thread, coating and any required test certification.
Coarse thread, fine thread and fit are not interchangeable details
Coarse pitch metric bolts are widely used because they are generally easier to assemble, more tolerant of minor contamination and less sensitive to thread damage. Fine pitch bolts can be useful where adjustment is important, where a reduced thread helix angle is desired, or where the design benefits from a larger tensile stress area for the same nominal diameter. However, fine threads are also easier to cross-thread, more sensitive to dirt and coating buildup, and may be less suitable for soft internal threads unless the engagement length is carefully designed.
Thread tolerance is another detail that becomes important in production. Common ISO metric thread fits use designations such as 6g for external threads and 6H for internal threads. Tolerance affects allowance and fit, especially after plating or galvanizing. Hot dip galvanizing, for example, adds more coating thickness than typical electroplating, so mating nut and thread allowances must be compatible. If a coated fastener binds before proper clamping is reached, the cause may be a specification mismatch rather than installer error.
Material and coating choices change performance
Carbon steel and alloy steel metric bolts are normally selected by property class, while stainless steel metric bolts are selected by stainless group and property class. A2-70 and A4-80 markings follow the ISO 3506 approach: A2 or A4 identifies the stainless steel group, while 70 or 80 indicates the strength class in hundreds of megapascals. A4 is often chosen for more demanding corrosion exposure than A2, but neither designation should be treated as a universal replacement for a carbon steel class 8.8 or 10.9 bolt. See also: Fittings & Valves.
Coatings add another decision point. Electroplated zinc and zinc-alloy coatings are common for indoor and moderate corrosion environments, and ISO 4042 covers fastener electroplating systems. That standard also addresses hydrogen embrittlement risk, a concern that becomes more important with high-strength steel fasteners. Hot dip galvanizing is thicker and can offer stronger barrier protection in outdoor steelwork, but ISO 10684 limits its fastener scope and states that hot dip galvanizing is not recommended for threaded fasteners below M8 or with pitches below 1.25 mm.
Stainless steel can reduce red rust but may suffer from galling, especially when stainless bolt and nut threads slide under load without suitable lubrication or material pairing. For this reason, stainless assemblies often need controlled lubrication, compatible nuts and realistic installation torque rather than a generic torque value copied from a carbon steel chart.
Specifying and buying metric bolts without avoidable errors
The most common ordering mistakes come from incomplete descriptions. “M12 bolt” is not enough information for a controlled purchase. A better purchase description should include diameter, pitch, length, head style, thread length or full-thread requirement, product standard, property class, material, coating, nut standard and washer requirement when applicable.
- Confirm the mating thread. Check the pitch of the nut or tapped hole before ordering replacement parts.
- Do not upgrade strength blindly. A 12.9 bolt is stronger in tensile terms, but it may be less forgiving in some joints and more sensitive to improper coating or installation.
- Match nuts to bolts. A high-strength bolt with an under-specified nut can strip the internal thread before the bolt develops the intended clamp load.
- Control the coating system. Plating, galvanizing and stainless material choices affect thread fit, torque, corrosion performance and embrittlement risk.
- Treat torque tables as starting points. Torque-to-tension results change with lubrication, surface finish, washer use and thread condition.
- Separate general fasteners from structural fasteners. Structural steel bolting may require ASTM, EN, ISO or project-specific assemblies and inspection procedures.
For maintenance teams, a small reference chart for common sizes, coarse pitches and property classes can reduce mistakes. For engineers and buyers, the more reliable approach is to copy the controlled callout from the drawing or bill of materials rather than reconstructing the specification from memory.
Frequently asked questions
What does M8 mean on a metric bolt?
M8 means the bolt has an ISO metric thread with a nominal diameter of 8 mm. It does not, by itself, specify the pitch, length, strength class, head type or coating. A complete callout should include those details when fit or performance matters.
Are metric 8.8 bolts the same as SAE grade 8 bolts?
No. The names look similar, but they come from different systems. Metric 8.8 is an ISO property class, while SAE grade 8 is an inch-system automotive fastener grade. They should not be substituted by name alone because dimensions, thread form and mechanical requirements differ.
Is coarse pitch or fine pitch better?
Neither is universally better. Coarse pitch is usually easier to assemble and more tolerant of rough service. Fine pitch can be useful for adjustment, limited engagement conditions or specific joint designs, but it requires more care with thread cleanliness, coating thickness and mating parts.
Can stainless metric bolts replace 8.8 carbon steel bolts?
Not automatically. Stainless steel bolts use different material and property class designations, such as A2-70 or A4-80. They may improve corrosion resistance in some environments, but their strength, galling behavior and torque response are different from carbon steel class 8.8 bolts.
What information should be on a metric bolt purchase order?
A good purchase order should state the metric size, pitch, length, head style, product standard, property class or stainless designation, coating, nut and washer requirements, quantity and any certification needs. For controlled assemblies, use the exact callout from the drawing or project specification.
