Metric threads, the most widely used general-purpose screw threads globally, are defined by the ISO Metric Screw Thread standard (ISO 68). They are characterized by a symmetric V-shape profile with a 60° flank angle.
Designation System
A metric thread is designated by the letter M followed by its nominal major diameter in millimetres. If the pitch is not the standard "coarse" pitch, it is specified after the diameter, separated by an "x" (or a dash).
- M8: Refers to a coarse thread with a nominal diameter of 8 mm (and an implied standard pitch of 1.25 mm).
- M8x1: Refers to a fine thread with a diameter of 8 mm and a pitch of 1 mm.
Coarse vs. Fine Threads
Metric threads are available in two primary pitch types, which are not related to manufacturing quality.
- Coarse Threads:
- Feature a larger pitch (distance between threads).
- Are the more common and general-purpose type, widely available and suitable for most applications.
- Allow for quick assembly and disassembly.
- Fine Threads:
- Have a smaller pitch, meaning more threads per unit of length.
- Offer enhanced strength and better resistance to loosening from vibration.
- Require a longer thread engagement and are often found in automotive and aerospace applications.
