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Calculate the correct thread pitch and nominal bore size.
If there are six whole threads within the N Distance and if N =- 3/4″ then the calculation is 6 divided by 3/4 which equals 8 TPI (TPI = Thread per Inch)
Example: If the above fitting measurement at X is 1-7/8″ (47.6mm), then the NB Size is 1-1/2″ (38mm)
| Nominal Bore(NB) Pipe Size |
Approx. Measurement of OutsideDiameter of MALE THREAD |
Threads per inch |
Thread Pitch (Distance between each thread)  |
|||
| Imperial | Metric | Imperial | Metric | (TPI) | (mm) | (inch) |
| 1/8″ BSP | (6mm) | 9.5mm | (3/8″) | 28 | 0.91 | 0.036 |
| 1/8″ NPT | (6mm) | 9.5mm | (3/8″) | 27 | 0.94 | 0.037 |
| 1/4″ BSP | (8mm) | 12.7mm | (1/2″) | 19 | 1.34 | 0.053 |
| 1/4″ NPT | (8mm) | 12.7mm | (1/2″) | 18 | 1.41 | 0.056 |
| 3/8″ BSP | (10mm) | 15.9mm | (5/8″) | 19 | 1.34 | 0.053 |
| 3/8″ NPT | (10mm) | 15.9mm | (5/8″) | 18 | 1.41 | 0.056 |
| 1/2″ BSP | (13mm) | 20.7mm | (13/16″) | 14 | 1.81 | 0.071 |
| 1/2″ NPT | (13mm) | 20.7mm | (13/16″) | 14 | 1.81 | 0.071 |
| 3/4″ BSP | (20mm) | 26.2mm | (1-1/32″) | 14 | 1.81 | 0.071 |
| 3/4″ NPT | (20mm) | 26.2mm | (1-1/32″) | 14 | 1.81 | 0.071 |
| 1″ BSP | (25mm) | 31.8mm | (1-1/4″) | 11 | 2.31 | 0.091 |
| 1″ NPT | (25mm) | 31.8mm | (1-1/4″) | 11-1/2 | 2.21 | 0.087 |
| 1-1/4″ BSP | (32mm) | 41.3mm | (1-5/8″) | 11 | 2.31 | 0.091 |
| 1-1/4″ NPT | (32mm) | 41.3mm | (1-5/8″) | 11-1/2 | 2.21 | 0.087 |
| 1-1/2″ BSP | (38mm) | 47.6mm | (1-7/8″) | 11 | 2.31 | 0.091 |
| 1-1/2″ NPT | (38mm) | 47.6mm | (1-7/8″) | 11-1/2 | 2.21 | 0.087 |
| 2″ BSP | (50mm) | 58.7mm | (2-5/16″) | 11 | 2.31 | 0.091 |
| 2″ NPT | (50mm) | 58.7mm | (2-5/16″) | 11-1/2 | 2.21 | 0.087 |
| 2-1/2″ BSP | (65mm) | 73mm | (2-7/8″) | 11 | 2.31 | 0.091 |
| 2-1/2″ NPT | (65mm) | 73mm | (2-7/8″) | 8 | 3.18 | 0.125 |
| 3″ BSP | (80mm) | 88mm | (3-15/32″) | 11 | 2.31 | 0.091 |
| 3″ NPT | (80mm) | 88mm | (3-15/32″) | 8 | 3.18 | 0.125 |
| 4″ BSP | (100mm) | 113mm | (4-15/32″) | 11 | 2.31 | 0.091 |
| 4″ NPT | (100mm) | 113mm | (4-15/32″) | 8 | 3.18 | 0.125 |
| 6″ BSP | (150mm) | 163mm | (6-15/32″) | 11 | 2.31 | 0.091 |
| 6″ NPT | (150mm) | 163mm | (6-15/32″) | 8 | 3.18 | 0.125 |
