Which one is UNC or UNF preferred


19th January 2015

Dear classic car owners,

English and American classics are factory-fitted with screws and threads with inch sizes. Our metric nuts and bolts look the same, but they don't fit our beloved Americans and British.

Inch screws are commercially available, but it can sometimes be annoying when you get the screw you want but have to take a whole package with 100 pieces. With us you get everything in Small quantities or in practical assortments in different sizes. Most of the inch screws are with us in stock available.


The 1 x 1 of the inch threads

The inch threads are not that difficult to understand:

Basically we differentiate between the English Whitworth and the American SAE threads.

A thread is determined by specifying the shaft diameter, thread pitch (distance between the threads) and the flank angle. This is how it is done with our metric threads.

With inch threads, you don't measure the distance between the threads, but the number of threads per inch (25.4mm) length. This is called the TPI (tread per inch).

This is how the name for inch screws can easily be explained.
E.g. 5/16 "-24 UNF x 1"  

  • 5/16 "is the thread diameter
  • 24 UNF means that our screw has 24 threads / inch (this corresponds to UNF)
  • 1 "is the thread length

Let's start with the Whitworth threads, although these have lost their importance because England changed over to the American SAE standard after 1945.

We differentiate between

  • BSW = British Standard Whitworth (coarse thread)
  • BSF = British Standard Fine (fine thread)

These threads have a flank angle of 55 °.

With the American SAE threads a distinction is made between:

  • NC = National Coarse (coarse thread)
  • NF = National Fine (fine thread)

As with metric threads, the flank angle is 60 °.

Since the SAE standard has been adopted in England, these threads are called UNC and UNF there.

  • UNC = Unified National Coarse and
  • UNF = Unified National Fine.

These screws can be found in English post-war vehicles, but also in old Swedish automobiles. The British preferred the UNF, while the Americans and Swedes preferred the UNC.

UNC thread

UNF thread

For screws, the outside diameter is specified in inches and the thread pitch in revolutions per inch.

For example: 1/4 "- 20 UNC

So if you want to determine a thread you need to determine if it:

A) is a UNC / UNF or a BSW / BSF thread (this gives the thread pitch)

B) determine the diameter


American thread table

For diameters under ¼ inch, the diameter is specified by a number defined in the standard from the following table. The designation (in the first column of the table) is spoken in English as "number", for example "number zero".

numberDiameter in inchesDiameter in mm


For all diameters from ¼ inch, the value is given in inches or fractions of inches. The gradations are common:

Diameter in inches (fraction)Diameter in Inch (decimal)Diameter in mm


You can find a PDF list of the most important UNF / UNC inch threads here: Inch thread table

You can find a PDF list of the most important English BSF / BSW inch threads here: Inch thread table

Of course you will not only find with us Inch screws, but also Hex nuts, lock nuts, washers and lock washers. Also practical assortments We have put together for you in different sizes. And everything in Small quantities!

We only sell high qualityquality for screws and hex nuts, which we also use in our own vehicles.


Our range of inch screws includes:

  • UNF / UNC screws and nuts galvanized steel grade 5 (8.8)
  • UNF / UNC screws and nuts steel yellow galvanized grade 5 (8.8)
  • UNF / UNC screws and nuts steel yellow galvanized grade 8 (10.9)
  • UNF / UNC screws and nuts stainless steel A2
  • BSF / BSW screws and nuts galvanized steel.
  • Thread cutting kits
  • Thread repair kits
  • Allen and Allen keys