3-8 NPT vs 3-1/2-8 NPT: specs compared
Updated 05 September 2026.
101.6 mm
3-1/2-8 NPT wins on major diameter
On major diameter, 3-1/2-8 NPT leads: 101.6 mm versus 88.9 mm.
| Spec | 3-8 NPT | 3-1/2-8 NPT |
|---|---|---|
| Designation | 3-8 NPT | 3-1/2-8 NPT |
| Major diameter | 88.9 mm | 101.6 mm |
| Threads per inch / pitch | 8 | 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 NPT or 3-1/2-8 NPT?
On major diameter, 3-1/2-8 NPT leads: 101.6 mm versus 88.9 mm.
How do 3-8 NPT and 3-1/2-8 NPT compare?
On major diameter, 3-1/2-8 NPT leads: 101.6 mm versus 88.9 mm.