3/8-18 NPT vs 5-8 NPT: specs compared
Updated 05 September 2026.
141.3 mm
5-8 NPT wins on major diameter
On major diameter, 5-8 NPT leads: 141.3 mm versus 17.15 mm.
| Spec | 3/8-18 NPT | 5-8 NPT |
|---|---|---|
| Designation | 3/8-18 NPT | 5-8 NPT |
| Major diameter | 17.15 mm | 141.3 mm |
| Threads per inch / pitch | 18 | 8 |
| Thread form | tapered, 60 degree | tapered, 60 degree |
| Seals on | on the thread flanks with PTFE tape | on the thread flanks with PTFE tape |
Questions and answers
Which has the higher major diameter, 3/8-18 NPT or 5-8 NPT?
On major diameter, 5-8 NPT leads: 141.3 mm versus 17.15 mm.
How do 3/8-18 NPT and 5-8 NPT compare?
On major diameter, 5-8 NPT leads: 141.3 mm versus 17.15 mm.