Guide to Selecting and Safely Using ISO Metric Bolts for Maintenance

Imagine a high-value precision piece of equipment rendered inoperable due to the failure of a single small bolt—the resulting losses could be catastrophic. In mechanical maintenance, bolts and nuts may seem insignificant, but they are critical to ensuring the safe and stable operation of equipment. However, with the vast variety of metric bolts and nuts available, how can one accurately identify, correctly select, and safely fasten them? This guide provides clear, practical instructions for these tasks.

In the assembly and maintenance of modern machinery, bolts and nuts play a vital role. They are the key components that connect various parts, ensuring the structural integrity of the equipment. At first glance, all metric bolts may appear similar, but in reality, they differ significantly in thread type, strength grade, and dimensional specifications. Incorrect selection or use can lead to equipment failure or even safety hazards. Therefore, a deep understanding of the characteristics of metric bolts and nuts, along with proper selection and fastening techniques, is essential for mechanical maintenance personnel.

This guide is designed for professionals working in factory and mechanical maintenance, providing fundamental knowledge about ISO metric bolts and nuts to help them correctly identify and use replacement parts, ensuring safe and reliable equipment operation. We will focus on the most commonly used metric bolts and nuts in industrial settings and offer practical tips for identification and application.

The ISO standard defines multiple specialized thread types, but in most mechanical equipment, only two are commonly used: standard pitch and fine pitch. Other thread types are primarily reserved for specialized devices such as precision instruments or optical equipment.

Pitch refers to the distance between adjacent threads, measured in millimeters. Standard pitch threads are suitable for most general-purpose connections, while fine pitch threads are used where higher locking force or vibration resistance is required.

The table below lists the most common combinations of thread diameters and pitches in industrial machinery:

With the exception of the lowest-grade bolts and nuts, all metric fasteners feature strength identification markings. These markings help quickly and accurately determine their load-bearing capacity.

Bolt heads are typically stamped with two numbers separated by a decimal point. The full range includes ten grades from 3.6 to 14.9, but in industrial settings, common grades are 8.8, 9.8, 10.9, and 12.9. The bolt head also includes a manufacturer code (usually two or three letters).

The first number indicates the tensile strength of the bolt steel, roughly measured in 10 kg per square millimeter of cross-sectional area. For example, a grade 12.9 bolt will withstand at least 120 kg per square millimeter before breaking. Similarly, a grade 8.8 bolt has a breaking load of 80 kg per square millimeter.

As a bolt gradually bears a load, it elongates slightly, like a spring. If the load is released, it returns to its original length. However, there is a limit beyond which the bolt will not fully recover and instead permanently stretch—this is the yield point. The second number on the bolt head represents the percentage of tensile strength the bolt can withstand before stretching begins.

For example, a grade 12.9 bolt begins to stretch when the load reaches 90% of its breaking load. Similarly, a grade 9.8 bolt starts stretching at 80% of its breaking load.

Strength markings on nuts are often harder to identify than those on bolts. They may appear as numbers or as patterns of dots and lines, where the line position resembles clock hands indicating the grade.

The nut grade should always match or exceed the bolt grade by one level. For example, a grade 8.8 bolt should pair with a grade 8 or 9 nut. A grade 12.9 bolt should pair with a grade 12 or 14 nut.

As mentioned earlier, when a bolt gradually bears a load, it reaches a yield point (where permanent stretching begins). If the load continues to increase, the bolt reaches its breaking point. Standards define a "proof load," typically 90% of the load required to stretch the bolt.

In practice, manufacturers often design machinery so that bolts bear loads well below the proof load, ensuring a robust safety factor.

Fine pitch bolts are typically 10% stronger than standard pitch bolts because less metal is removed during threading.

The recommended maximum torques in the table below assume lightly lubricated threads. Tightening a bolt to the specified torque should set the tension in the bolt to approximately 85% of the proof load, equivalent to 62% of the breaking load.

Use these torque values only when the machine manufacturer has not specified a torque setting.

Fine pitch bolts and nuts are typically used only in special cases, and the manufacturer's maintenance documentation should specify the required torque for each application.

Fine pitch threads are often used when bolts are screwed into soft metal blocks (e.g., aluminum castings), where the threads in the block may be much weaker than the bolt and usually require lower torque to avoid stripping.

Many factors influence the strength of threaded parts, including production methods, surface finish, and thread shape precision.

The strength of a threaded connection depends on its weakest component. For example, if a high-strength bolt is screwed into a softer, more ductile material (e.g., aluminum alloy or low-carbon steel), the softer part determines the maximum safe torque.

More commonly, bolts must withstand not only tensile loads from tightening but also lateral or "shear" loads. A typical example is two plates bolted together and subjected to forces that apply shear loads to the bolt.

If the applied load is very small, friction between the plates due to bolt tightness may prevent movement. However, unless dowel pins are installed to bear the shear load, the bolt will eventually experience both shear and tensile stress. Combining these stresses creates a greater total stress than either alone, so the bolt's pre-torque may need to be lower than the maximum values to accommodate additional stress from shear.

As a guideline, reducing the tightening torque by 10% allows 35% of the bolt's load rating to handle shear forces without overloading.

Design engineers must consider all these factors and may specify lower torque, especially in lifting equipment requiring high safety margins.

Always refer to the machine manufacturer's maintenance documentation for specified torque values for bolts and nuts in lifting and load-bearing applications.

The table below lists the correct wrench and hex key sizes for the most common bolt and nut sizes in industrial machinery.

To minimize the risk of wrench slippage and damage to fastener edges, use a box-end wrench (or the box-end of a combination wrench) whenever possible.

Metric bolts are manufactured slightly smaller than their nominal size—for example, an M16 bolt typically has a shank diameter of 15.97 mm. This means a 16 mm bolt can pass through a 16 mm hole, but to account for misalignment between parts, clearance holes are typically drilled slightly larger.

The table below provides typical clearance hole sizes and the correct tap drill sizes for standard and fine pitch threads.

For metric threads, tap drill sizes are easily calculated as the bolt diameter minus the thread pitch. For example, a standard pitch M16 bolt with a 2.0 mm pitch requires a tap drill size of 16 – 2 = 14 mm.

When tapping holes, especially for M10 and smaller sizes, frequently remove the tap to clear metal chips and prevent breakage.

When tightening a bolt into threads or a nut and bolt assembly to clamp parts together, the applied torque generates tensile stress in the bolt shank.

The actual force in the bolt depends on factors such as thread shape, pitch, surface finish, and lubrication. For most standard cases with lightly lubricated metric bolts, the force can be approximated as:

Force = 5 × Torque / Diameter

Where force is in newtons (N), torque in newton-meters (Nm), and diameter in meters (m). For example, an M16 bolt tightened to 247.5 Nm generates:

Force = 5 × 247.5 Nm / 0.016 m = 77,344 N (77.3 kN)

To convert newtons to kilogram-force (kgf), divide by 9.81 (or 10 for quick estimates), yielding 7,884 kgf of bolt load.

Dry threads increase friction, resulting in lower force.

Bolted joints typically fail in three ways: shear failure, tensile fracture, or thread stripping.

Tensile fracture (bolt snapping) occurs from joint overload or excessive tightening. Bolts often fail where the thread meets the shank.

Shear failure happens when lateral forces exceed the bolt's capacity, often exacerbated by over-tightening leaving no strength reserve for shear loads. Both failure types usually occur early or late in a component's lifecycle.

A frequent service failure involves lateral loads prying nuts off bolts, causing cascading failures. Thread taper leads to nut expansion, reducing thread engagement until metal yields and threads strip.