Thread standards you should know

Thread standards ensure that components remain interchangeable and that your work results are reproducible at all times. This is particularly important if you want to cut threads yourself. Which thread standard should the tap have and why does your spare part not fit as you had imagined? As a manufacturer of threading tools with 40 years of company history, we at BAER hear these and similar questions every day. To prevent confusion and clarify the most important key data, we have compiled an overview of the most common thread standards for you

You should already have heard of these thread types

There is an abundance of thread standards - from widely used standard threads to obscure special cases. However, to help you find your way around in your daily work as a DIY enthusiast or craftsman, a few thread standards are sufficient as a guide.

The metric ISO thread (M)

As this type of thread is used very frequently in Europe, you have probably already worked with it yourself. The thread standard for the metric standard thread is a fastening thread that is used for screws and nuts, for example. It has a flank angle of 60° and is characterised by the abbreviation M and the nominal diameter. As this is a metric thread standard, all dimensions are given in mm.

The metric ISO fine pitch thread (MF)

This type of thread is a variant of the metric ISO standard thread with a smaller pitch and thread depth. The small pitch of the MF metric fine thread makes it possible to determine the position of a screw very precisely. The thread standard is therefore often used for adjusting screws. In addition, fine threads with this standard have good self-locking properties and can also be used in tight spaces thanks to their shallow depth. The abbreviation MF is used in the thread designation.

The trapezoidal thread (Tr)

The trapezoidal thread is also a metric thread standard; accordingly, all Dimensions are specified in mm. The flank angle here is only 30°. A striking feature of this thread is the profile, which is not tapered, but has the shape of an isosceles trapezoid. As a result, the trapezoidal thread is well suited as a movement thread and can be found in lathes or screw presses, for example.

Unified National Coarse Thread (UNC)

At this point we enter the area of American thread types. With a flank angle of 60°, the American coarse thread is similar to the metric ISO thread and is also roughly equivalent to this thread standard in the USA in terms of function and distribution. It is important to note that all dimensions are given in inches. In addition, the pitch of American threads is specified by the number of threads in inches. A small but important detail that can be confusing at first glance.

Unified National Fine Thread (UNF)

As with metric threads, there is also a fine thread to the coarse thread for American threads. This is the Unified National Fine Thread (UNF). The flank angle of the American fine thread is 60° and here, too, all specifications must be in inches. Like the UNC thread standard, the UNF thread standard is also used worldwide and is also used in the USA, Japan and Australia, among others.

Unified National Special Thread (UNS)

The UNS thread standard is a special standard for threads. This standard also has a flank angle of 60°, but differs from the other American threads in its special diameters, pitches and screw-in lengths. This American thread with a special pitch can be found on microphone stands, for example.

National Standard Taper Pipe (NPT)

Although the NPT thread is also one of the American thread standards, it is not part of the Unified Thread Standard (UTS). Rather, it belongs to a group of standards for pipe threads. The American pipe thread is tapered, non-sealing and has a flank angle of 60°.

British Standard Whitworth Coarse Thread (BSW)

Let's continue with the British threads. The British coarse thread is the standardised standard thread in the UK, similar to the metric thread standard on the European mainland. The dimensions are given in inches. Be careful with the flank angle, which is only 55° here. The thread abbreviation for this thread standard is made up of the abbreviation BSW and the nominal diameter in inches.

British Standard Fine Thread (BSF)

The British thread standards also recognise the fine thread for the BSW thread. The British Standard Fine Thread (BSF) has a flank angle of 55°. The British fine thread used to be an important part of the glorious British automotive industry, but has since lost its importance. You can recognise this thread by the thread designation BSF followed by the diameter in inches.

British Standard Pipe Parallel (BSP)

While the other British thread standards are mainly used in their country of origin, the BSP pipe thread is also very popular outside the UK. It has established itself as the standard thread for connecting pipes in plumbing and heating technology and can therefore also be found in Germany. Please note that this thread standard also has a flank angle of 55°. In addition, the inches in the thread designation should not be taken literally, as they refer to obsolete pipe geometries. The abbreviation for the BSP thread is G.