All of our screw threads are supplied as BSP threads unless otherwise specified.

It can be confusing telling which thread size is which from an external measurement of the thread. This is because they measure differently from their named thread size. The guide below is to help you to quickly identify which size thread you have.

If Measuring the Outside of a Male BSP Thread (called Actual Major Diameter)

  • If it is 9.70mm External Diameter — then the actual thread size is called a 1/8" BSP Thread
  • If it is 13.1mm External Diameter — then the actual thread size is called a 1/4" BSP Thread
  • If it is 16.6mm External Diameter — then the actual thread size is called a 3/8" BSP Thread
  • If it is 21.0mm External Diameter — then the actual thread size is called a 1/2" BSP Thread
  • If it is 26.4mm External Diameter — then the actual thread size is called a 3/4" BSP Thread
  • If it is 33.2mm External Diameter — then the actual thread size is called a 1" BSP Thread
  • If it is 42.0mm External Diameter — then the actual thread size is called a 1 ¼” BSP Thread
  • If it is 47.8mm External Diameter — then the actual thread size is called a 1 ½” BSP Thread
  • If it is 59.6mm External Diameter — then the actual thread size is called a 2” BSP Thread

Measure the Inside of a Female Thread Port:

  • If it is 8.90mm Internal Diameter — then the actual port size is a Female 1/8" BSP Thread
  • If it is 11.9mm Internal Diameter — then the actual port size is a Female 1/4" BSP Thread
  • If it is 15.2mm Internal Diameter — then the actual port size is a Female 3/8" BSP Thread
  • If it is 19.1mm Internal Diameter — then the actual port size is a Female 1/2" BSP Thread
  • If it is 24.6mm Internal Diameter — then the actual port size is a Female 3/4" BSP Thread
  • If it is 31.0mm Internal Diameter — then the actual port size is a Female 1" BSP Thread
  • If it is 39.6mm Internal Diameter — then the actual port size is a Female 1 ¼” BSP Thread
  • If it is 45.5mm Internal Diameter — then the actual port size is a Female 1 ½ ” BSP Thread
  • If it is 57.4mm Internal Diameter — then the actual port size is a Female 2” BSP Thread

 

Inch Size

Male Thread O.D. mm

Female Thread I.D. mm

1/8

9.7

8.9

 1/4

13.2

11.9

3/8

16.5

15.2

 1/2

20.8

19.1

5/8

22.4

20.3

3/4

26.4

24.6

1

33.0

31.0

1 ¼

41.9

39.6

1 ½

47.8

45.5

2

59.7

57.4