Internal thread – Female thread - chart, how to make, how to measure

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What is an internal thread?

Internal thread is a thread that is inside the hole, e.g., within the nut. The thread coil is in the centre of what you screw a male thread (which is usually a screw) into.

However, we can list more examples of this type of thread. In addition to nuts, these are:

  • internal threads in pipes, 
  • pipe fittings, 
  • threads drilled into a metal or wooden structure using a machine tap,

Internal threads are also often referred to as the female variety of thread.

How do you make an internal thread?

There are three methods of making internal threads:

  • tapping,
  • kneading,
  • milling.

Tapping is the cheapest and easiest option, which is why it is the most popular. To perform it, you need a tapping tool, i.e. a special drill for thread-cutting.

Unfortunately, this method is characterised by a lower surface quality and has problems with hard-to-machine materials.

As far as kneading is concerned, this is chipless machining. Threads are formed by progressively deforming the surface with a forming tap. This method does not produce chips and has a higher quality of threads produced, but is not suitable for materials not susceptible to plastic deformation.

We are still left with milling, which offers the greatest quality and multifunctionality, but is also the most costly (tool price) and complex.

With milling cutters you will produce very deep threads of various sizes even in hardened materials.

 In addition, they can also be used to produce external threads.

What are the dimensions of internal threads? Internal threads dimension chart

The chart below indicates all the necessary dimensions for standard internal threads that you will find on the market:

Inner diameter[mm]Coilsper inchPitch[mm]Inch Pipe BSP BSPTMetricInch UNF [JIC, ORFS]Inch NPT, NPTF
6,8 – 7,0 1 M8x1  
8,5 – 8,928[0,91]1/8”   
8,5 – 8,927[0,95]   1/8”
8,2 – 8,6 1.5 M10x1,5  
8,7 – 9,1 1 M10x1  
9,7 – 10,020[1,27]  7/16” - 20 
10,2 – 10,6 1.5 M12x1,5  
11,3 – 11,620[1,27]  1/2” - 20 
11,4 – 11,919[1,34]1/4”   
11,4 – 11,918[1,41]   1/4”
12,2 – 12,6 1.5 M14x1,5  
12,7 – 13,018[1,41]  9/16” - 18 
14,2 – 14,6 1.5 M16x1,5  
14,9 – 15,419[1,34]3/8”   
14,9 – 15,418[1,41]   3/8”
15,8 – 16,116[1,59]  11/16” - 16 
16,2 – 16,6 1.5 M18x1,5  
17,3 – 17,616[1,59]  3/4” - 16 
18,2 – 18,6 1.5 M20x1,5  
18,9 – 19,316[1,59]  13/16” - 16 
18,6 – 19,014[1,81]1/2”   
18,3 – 18,714[1,81]   1/2”
20,2 – 20,6 1.5 M22x1,5  
20,2 – 20,514[1,81]  7/8” - 14 
20,6 – 21,014[1,81]5/8”   
22,2 – 22,6 1.5 M24x1,5  
23,4 – 23,814[1,81]  1” - 14 
24,2 – 24,6 1.5 M26x1,5  
24,1 – 24,514[1,81]3/4”   
23,7 – 24,114[1,81]   3/4”
24,3 – 24,712[2,12]  1.1/16” - 12 
24,6 – 25,0 2 M27x2  
25,2 – 25,6 1.5 M27x1,5  
26,2 – 26,6 1.5 M28x1,5  
27,4 – 27,8 2 M30x2  
28,2 – 28,6 1.5 M30x1,5  
27,6 – 27,912[2,12]  1.3/16” - 12 
27,8 – 28,114[1,81]7/8”   
30,5 – 30,9 2 M33x2  
31,2 – 31,6 1.5 M33x1,5  
30,3 – 30,811[2,31]1”   
30,8 – 31,212[2,12]  1,5/16” – 12 
30,3 – 30,811.5[2,21]   1”
33,4 – 33,8 2 M36x2  
34,3 – 34,712[2,12]  1,7/16” -12 
34,8 – 35,111[2,31]1.1/8”   
36,2 – 36,6 1.5 M38x1,5  
38,7 – 39,112[2,12]  1.5/8” - 12 
39,4 – 39,8 2 M42x2  
39,0 – 39,511[2,31]1.1/4   
39,2 – 39,611.5[2,21]   1.1/4”
40,6 – 41,012[2,12]  1.11/16” - 12 
42,4 – 42,8 2 M45x2  
43,2 – 43,6 1.5 M45x1,5  
45,1 – 45,512[2,12]  1.7/8” - 12 
44,8 – 45,311[2,31]1.1/2”   
45,1 – 45,511.5[2,21]   1.1/2”
48,6 – 49,012[2,12]  2” - 12 
49,4 – 49,6 2 M52x2  
50,2 – 50,6 1.5 M52x1,5  
56,5 – 56,811[2,31]2”   
56,4 – 56,711.5[2,21]   2”
61,3 – 61,812[2,12]  2.1/2” - 12 
62,6 – 63,0 2 M65x2  
62,7 – 63,011[2,31]2.1/4”   
68,8 – 69,18[3,17]   2.1/2”
72,2 – 72,511[2,31]2.1/2”   
84,9 – 85,311[2,31]3”   
84,7 – 85,18[3,17]   3”
110,1 – 110,511[2,31]4”   
110,2 – 110,68[3,17]   4”
123,2 – 124,0 6 M130x6  
135,5 – 135,911[2,31]5”   
127,5 – 127,9 9.7  5,1/2” (DIN11) 
 8[3,17]   5”
160,9 – 161,411[2,31]6”   
 8[3,17]   6”
 8[3,17]   8”
Internal thread examples - Female thread examples

How to measure an internal thread with a calliper?

You will measure the internal threads with the calliper using its internal jaws, Simply insert them into the threaded hole, expand to the maximum and then read the result.

Then you can use the chart above to find the type of thread you are dealing with.

As it gives the dimensions in millimetres, you can easily check which thread you are dealing with (even if it is, for example, a 1 inch internal thread).

How to fix an internal thread?

You can easily repair a machined or damaged internal thread with a machine tap. However, use this tool with your head, as its use will lead to a widening of the hole and its coiling.

Unfortunately, this cannot be avoided.

There is another solution - special springs made of spring steel. They function in such a way that once you have widened the hole with the machine tap, you reach for the screwdriver and screw the corresponding spring into the centre.

Despite appearances, reconditioning threads does not make them less durable. In fact, in some cases, the opposite may be true - the process will make them even more resistant and durable.

Photo of Bartosz Kułakowski - CEO of Hosetech sp. z o. o.

Articles author

Bartosz Kułakowski

CEO of Hosetech Sp. z o.o.

Bartosz Kulakowski is an industrial hoses and couplings specialist with over 10 years of experience. Bartosz has been present in the technical industry since 2013. He gained experience as a technical and commercial advisor in the sector of plastic conveyor belts, steel structures, industrial hoses, and connectors. Since 2016, he has specialized exclusively in hoses and connectors. In 2019 he opened his own business under the HOSETECH Bartosz Kulakowski brand and since July 2022 he has been the CEO of the capital company HOSETECH Sp z o. o. (LLC).

SPIS TREŚCI

What is an internal thread?

How do you make an internal thread?

What are the dimensions of internal threads? Internal threads dimension chart

How to measure an internal thread with a calliper?

How to fix an internal thread?

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