Identifying the size of a thread

Date Posted:28 January 2014 

There are many different thread standards used in the engineering industry but by far the most common in Australia and any former Commonwealth Country is BSP (British Standard Pipe). In this post we will help you identify what size thread you have.

A common mistake is to simply measure the inside diameter of the pipe and assume that this measurement will be the thread size you are after. For example if you measure 25mm you may think that this is 1″ BSP but in fact it is really 3/4″ BSP as this measurement fits between the major and minor diameter for this size. You really need to think of the thread size required as the ‘trade size’ as this is what the thread is known as in the industry (not a direct correlation to the size you measure). I have copied a thread size table off our web page (https://www.valvesonline.com.au/references/threads) to help you establish what size thread you have. If you measure the id of your pipe find where that measurement falls between the minor and major diameter. This is then your size (All dimensions in mm).

Trade Size (BSP)

Pitch

 

Major Diameter

 

Minor Diameter

 

Gage Length

 

1/8

0.907

9.728

8.565

3.97

1/4

1.337

13.157

11.445

6.012

3/8

1.337

16.662

14.95

6.35

1/2

1.814

20.955

18.633

8.164

3/4

1.814

26.441

24.12

9.525

1

2.309

33.249

30.292

10.391

1 ¼

2.309

41.91

38.953

12.7

1 ½

2.309

47.803

44.846

12.7

2

2.309

59.614

56.657

15.875

2 ½

2.309

75.184

72.227

17.463

3

2.309

87.884

84.927

20.638

4

2.309

113.03

110.073

 

Identifying a thread size

 


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