Thread types - Metric - UNC - UNF - NPT

• Posted on 6 February 2020
• By Gerard "Kollie" Kolsteren
• Posted in Metric Thread, NPT Thread, Thread, Thread predrill diameter, Threads, UNC Thread, UNF Thread
• 0

There are many kinds of threads. Metric threads will be familiar to most people. But on American Motorcycles, UNC threads and UNF threads are often used. Also NPT threads can be encountered sometimes. And then it gets a bit confusing. But in the end it is not that bad.

In this blog I write about the different types of threads you can encounter while rebuilding / customizing your custom motorcycle. 
Most of you will be familiar with the metric threads like M6, M8 and for example M12. This thread type is called Metric and is used a lot in Europe.
If you look at the table below you can see what the outer diameter (nominal diameter) of the thread is and what the drilling diameter is for internal metric threads. With metric bolts it is very easy to determine which bolt it is. An M6 bolt is 6mm thick, an M12 bolt is 12mm thick and an M20 bolt is 20mm thick. There is also a column with "Pitch". Pitch means no more than the displacement in the thread at a 360° rotation. So if you mount an M6 bolt in an M6 hole and you rotate the bolt 360° (a full rotation) then the M6 bolt moves 1mm. In the webshop we sell several bolts and nuts with Metric, UNC and UNF threads.

A drilling diameter is also indicated in the table below. You can also calculate these yourself if you know the pitch.

Outer diameter - Pitch = Drill diameter

Metric Thread

In addition to the standard metric thread, there is also "Metric FINE thread".

Metric FINE thread

UNC and UNF threads no longer work with millimetres (mm) but with inches ("). This is because it is American thread and they work with the Imperial System. UNC threads are suitable for the most universal applications. UNF threads are suitable for threads in sheet metal and are suitable for fine adjustment.

If you now want to know the outside diameter in millimeters, you can convert this:
Example: 1/4" x 20 UNC -> 1 inch is 25.4 mm -> 25.4 x 1/4 = 6.35 mm

Do you want to calculate the Pitch?
Example: 1/4" x 20 UNC -> 1 inch is 25.4 mm -> 20 is the number of revolutions per inch -> 25.4 / 20 = Rush 1.27 mm

To calculate the pre-drill diameter you do this:
Example: 1/4" x 20 UNC -> outer diameter (1/4 x 25.4 = 6.35 mm) minus Pitch (25.4 / 20 = 1.27 mm) -> 6.35 mm - 1.27 mm = 5.08 mm

UNC Thread (Unified National Coarse)

UNF Thread (Unified National Fine)

NPT thread is conical thread. This thread is used for liquid or gas.
This thread seals well because of the conical shape. These are some examples.

NPT/NPTF Thread (American Taper Pipe / National Pipe Taper Fuel)