The 'M' numbers on bolts denote their metric sizing, specifically indicating the nominal major diameter of the bolt's thread in millimeters. This system is part of the widely adopted ISO metric screw thread standard, which ensures interchangeability and compatibility of fasteners globally.
When you see a bolt labeled "M8," for instance, the "M" signifies that it is a metric thread, and the "8" indicates that its major diameter is approximately 8 millimeters. This number is a fundamental aspect of identifying and specifying metric bolts, helping users select the correct fastener for a given application or a compatible nut and threaded hole.
Understanding Metric Bolt Dimensions
Metric bolts are categorized based on their thread dimensions, with the "M" number being the primary identifier for the major diameter. This diameter is the largest diameter of the screw thread, measured from the crest of one thread to the crest of the opposite thread.
Here's a breakdown of common metric thread sizes and their corresponding major diameters, along with typical clearance hole diameters which are often considered when drilling for these bolts:
| Thread Size | Major Diameter (mm) | Clearance Hole Diameter (mm) |
|---|---|---|
| M5 | 5.0 | 5.8 |
| M6 | 6.0 | 7.0 |
| M7 | 7.0 | 8.0 |
| M8 | 8.0 | 10.0 |
- Thread Size (e.g., M8): This is the nominal designation for the metric thread.
- Major Diameter (mm): This is the actual outside diameter of the threaded portion of the bolt, measured in millimeters. This is the core dimension indicated by the number following the 'M'.
- Clearance Hole Diameter (mm): This refers to the recommended diameter for a hole that the bolt can pass through freely without engaging the threads. It is typically slightly larger than the bolt's major diameter.
Beyond the 'M' Number: Other Key Dimensions
While the 'M' number provides the major diameter, a complete metric bolt specification also includes other critical dimensions:
- Thread Pitch: This is the distance between adjacent thread crests, measured in millimeters. For example, an M8 bolt might have a standard pitch of 1.25mm (M8x1.25) or a fine pitch (e.g., M8x1.0). If no pitch is specified, it generally refers to the standard coarse pitch for that diameter.
- Length: The length of the bolt is measured from under the head to the tip of the bolt (for most bolt types).
For more detailed information on metric thread dimensions and bolt sizing, you can refer to resources like the Accu Metric Thread Chart. Understanding these specifications is essential for proper fastening, ensuring secure and reliable connections in various mechanical and structural applications.
