Thread types - Metric - UNC - UNF - NPT

What thread types are there? Metric threads, UNC threads, UNF threads?

Still useful to know what threads there are when you are going to change your custom motorcycle.

In this article you will read about the different types of threads you can come across when rebuilding/customizing your chopper or bobber bike. 

Most of you will know the metric threads. M6, M8 and for example M12. This thread type is called metric because it comes from the metric system. Almost the entire world uses the metric system except for the United States, Myanmar and Liberia.

If you look at the thread table below you can see what the outside diameter (nominal diameter) of the thread is and what the drill 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.

In the thread table there is also a column with the word "Pitch".
Pitch means no more than the displacement in the thread on a 360° rotation. So simply put, if you screw an M6 bolt into an M6 hole and you rotate the bolt 360° (one complete revolution) the M6 bolt will move 1mm in the nut.

There is also a (pre) drilling diameter indicated in the table below.
This predrill diameter you can also calculate yourself if you know the pitch. You then use the formula below:

Outer diameter (Ø) - Pitch (P) = Drill Size

Thread Table Metric threads with predrilling diameter

In addition to the standard metric thread, there is also "Metric Fine Thread".
Metric fine thread, like the regular metric thread, has an outside diameter that corresponds to the name of the thread.
The outer diameter of M10x0.75 is simply 10mm. All the pitch is different. With each revolution, the bolt or nut in this case moves 0.75mm.
You can actually see it. The teeth also look much finer. The teeth are smaller.

Thread Table Metric Fine Threads with Predrilling Diameter

What is the difference between UNC and UNF threads?

UNC and UNF threads no longer use millimeters (mm) but inches (").
This is because they are American threads and they work with the Imperial system as opposed to the for us more familiar metric system.

• UNF threads are suitable for threads in sheet metal and are suitable for fine adjustment.
  
  
• UNF thread has a smaller helix angle and better adhesion, and is more conducive to thread self-locking. Therefore, it is not as easy to loosen compared to UNC thread. It is generally used for thin-walled parts and parts with higher vibration damping requirements.
  
  
• UNF thread has a smaller pitch, there are more teeth in the same thread length, it can reduce fluid leakage, so it is used in applications where sealing is required.
  
  
• The UNC thread is suitable for the most universal applications.
  
  
• The UNC thread has fewer teeth of the same length, larger cross-sectional size of each tooth, and more suitable for carrying larger tensile and impact force.
  
  
• Compared with fine thread, coarse thread has higher strength and good interchangeability.

If you want to know the outside diameter of a UNC bolt in millimeters you can convert it that way:

• Example:
  • 1/4" x 20 UNC
  • 1 inch is 25,4 mm
  • 25,4 x 1/4 = 6,35 mm
    
    

So the outside diameter of a 1/4" bolt is 6.35mm.

Want to calculate the pitch of UNC or UNF threads?

• Example:
  • 1/4" x 20 UNC
  • 1 inch is 25,4 mm
  • 20 is the number of revolutions per inch
  • 25.4 / 20 = pitch 1.27 mm
    
    

So if you turn a 1/4" x 20 UNC bolt into a nut and you turn the bolt 360° the bolt moves 1.27mm.

Calculate from the pre-drill diameter of UNC and UNF threads:

• Example:
  • 1/4" x 20 UNC
  • Outer diameter (1/4 x 25.4 = 6.35 mm) minus Fluid (25.4 / 20 = 1.27 mm)
  • Outside diameter 6,35 mm - 1,27 mm = 5,08 mm
    
    

So if you want to make internal thread 1/4" x 20 UNC then you will need to drill a hole of Ø5.08mm (rounded off Ø5.1mm) before turning a threaded tap into the hole.

Thread Table UNC Threads with Pre-drilled Diameter

(Unified National Coarse)

Thread Table UNF Threads with Predrilling Diameter

(Unified National Fine)

Thread Table NPT/NPTF threads with predrilling diameter

(American Taper Pipe / National Pipe Taper Fuel)

NPT threads are conical threads. Conical means that the diameter of the thread is not the same thickness everywhere.
This thread is used for liquid or gas. This thread seals well due to its conical shape.

We supply various hose barbs with NPT threads and we also sell welding bungs with NPT threads.

Are you still looking for Metric, UNC or UNF bolts, nuts or washers? 

You can buy them each in the online webshop of Kollies Parts.