Metric and Imperial Threads: A Guide

  • 12 March 2024
  • guides

Threading standards are a cornerstone of engineering and manufacturing, and understanding the differences between metric and imperial threads is crucial for compatibility and strength in your projects.

Metric Threads

As the name suggests, metric bolts are measured in millimetres. The metric system offers two primary thread types: coarse and fine.

Metric bolts are identified by measuring the distance between the threads - a 1mm pitch bolt has 1mm between the threads.

For metric bolts, the prefix 'M' is added to denote Metric. For example, a 4mm diameter bolt is commonly known as an 'M4' bolt.

Metric Coarse

Metric coarse threads, characterised by their larger pitch (distance between threads), are a staple in general applications where quick assembly and disassembly are valued.

Metric Fine

Metric fine threads, with their increased thread count per unit length, provide enhanced strength and are preferred in applications requiring greater precision and load-bearing capacity, such as in machine screws.

Imperial Threads

Imperial bolts are measured in inches. The thread pitch of an imperial bolt is denoted by the number of threads per inch.

BSW and BSF: The British Standards

The imperial threading system, which originated in the British Industrial Revolution, presents a fascinating history of innovation and standardisation.

British Standard Whitworth (BSW) is one of the earliest standardised thread forms, introduced by Joseph Whitworth in the 19th century. Its coarse pitch made it ideal for the manufacturing demands of the time.

British Standard Fine (BSF) was developed as a finer alternative to BSW, offering better tensile strength. It was predominantly used in the automotive and engineering sectors in the UK until more universal thread forms superceded it.

UNF and UNC: Unifying Threads Across Nations

The onset of World War II marked a pivotal moment in thread standardisation with the introduction of the Unified Thread Standard, encompassing both Unified National Fine (UNF) and Unified National Coarse (UNC) threads. This system was devised in 1949 to facilitate the interoperability of machinery and equipment manufacturing among the USA, Canada, and Great Britain.

Unified National Fine (UNF) threads, being finer, are preferred for their higher tensile strength.

Unified National Coarse (UNC) threads are used in applications requiring fast assembly and robustness.

Pipe Threads - BSP, BSPT, NPT & NPTF

The plumbing and pipe fitting world introduces two specialised thread types: British Standard Pipe (BSP) and National Pipe Thread (NPT). While BSP is widely adopted globally, North America favours NPT. Both serve the critical function of ensuring leak-proof connections in plumbing systems.

NPTF (National Pipe Taper Fuel) threads provide an interference fit that ensures a tight seal for applications demanding a guaranteed seal without additional sealing compounds.

British Standard Pipe Taper (BSPT) threads offer a tapered profile for a secure connection, making them ideal for fluid systems where leak prevention is paramount.

BA Threads

The British Association (BA) thread has diameters of 6mm (0BA) and smaller, and was - and still is - particularly used in precision machinery.

Despite imperial origins, British Association (BA) threads have dimensions that correspond more closely to metric measurements. With diameters starting at 6mm (0BA) and shrinking down for smaller sizes, these threads have been widely used in precision engineering, particularly in instruments and model-making.

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