NPSI is a complimentary thread series part of the Dryseal Pipe thread standard defined in ASTM B1.20.3.
The typical dryseal thread is the NPTF, tapered thread. However, in some cases, it is desired to mate the external tapered NPTF thread with an internal straight thread.
- NPSI threads are straight (cylindrical) instead of tapered.
- They are internal only, intended to mate with external tapered PTF-SAE short threads.
- They have slightly larger diameters than the NPSF threads.
- They are more economical to manufacture than tapered threads.
- Sealing is not guaranteed since root and crest interference will not occur for all threads.
- NPSI threads are used with hard/brittle materials that can adjust at assembly to the external taper during assembly.
- Since they are cylindrical, the definition of pitch, major and minor diameters are more in-line with the methods used in regular metric or inch threads.
NPSI Thread Dimensions chart
| Thread | Pitch Diameter | Min. Minor Diameter | Min. Thread Length |
|---|---|---|---|
| 1/16" - 27 NPSF | 0.2791 - 0.2826 | 0.2505 | 0.31 |
| 1/8" - 27 NPSF | 0.3715 - 0.375 | 0.3429 | 0.31 |
| 1/4" - 18 NPSF | 0.4886 - 0.4938 | 0.4457 | 0.47 |
| 3/8" - 18 NPSF | 0.624 - 0.6292 | 0.5811 | 0.5 |
| 1/2" - 14 NPSF | 0.7745 - 0.7812 | 0.718 | 0.66 |
| 3/4" - 14 NPSF | 0.985 - 0.9917 | 0.9283 | 0.66 |
| 1" - 11 1/2 NPSF | 1.2338 - 1.242 | 1.1655 | 0.78 |
- Dimensions in Inches
