A guide to the various thread types found on fasteners in the UK

Metric Coarse

One of the commonest thread types found on metric fittings. Coarse threads have a larger pitch than fine threads, meaning that they have fewer threads per axial distance (distance along the thread).

Metric fine

Metric fine, another common thread, is often found on machine screws. More threads per axial distance than metric coarse. A fine thread is known to be stronger than a coarse thread.

BSW (British Standard Whitworth)

An imperial coarse thread first used in standard production by Joseph Whitworth during the industrial revolution. Whilst it is still used in engineering applications all across the world it is less common in the UK where metric threads have generally taken over.

BSF (British Standard Fine)

British Standard Fine thread was introduced as a fine-pitch alternative to BSW and commonly used in engineering and automotive applications in the UK. It was superseded by the UNF thread type in the 1940’s.

UNF (Unified National Fine)

Unified thread types (UNF and UNC) were introduced at the beginning of World War 2 to enable the interchangeability of machinery manufacturing between the USA, Canada and Great Britain. Still commonly found on a vast array of old machines and vehicles in the UK, UNF and UNC threads continue to be the standard thread in the USA and Canada.

UNC Unified National Coarse

The coarse thread equivalent of the UNF thread described above, with less threads per axial distance than the UNF equivalent.

BSP (British Standard Pipe)

BSP is used in the UK and all over the world as a standard thread for the plumbing and pipe fitting industry. Only North America does not use this thread, using instead the NPT thread (National Pipe Thread).

BSPT (British Standard Pipe Taper)

A variation of BSP with a tapered rather than parallel thread, with the diameter increasing or decreasing along the length of the thread. A taper male thread will mate with either a parallel or a taper female thread, but a taper female thread will only accept a taper male thread. Taper threads are denoted by the letter R.

NPT (National Pipe Thread)

This thread is the USA equivalent of BSP. It is used in North America and in various locations worldwide.

NPTF (National Pipe Taper Fuel)

This American thread was designed to create a leak-free seal without the need for PTFE or any other jointing compound. This is achieved because male and female NPTF threads are an interference fit. This is an engineering term that means there is an overlap between the maximum size of the male thread and the minimum size of the female thread, i.e. the external male NPTF thread external dimension is slightly larger than the internal female thread dimension.

BA (British Association)

A rarely used thread that was once commonly found on small instruments and models. Still sparingly used in some industries such as metering equipment manufacturing and optical equipment.