3-1/2-8 NPT taper Thread Dimensions
3-1/2-8 NPT is a pipe National Pipe Taper thread. There are 8 threads per inch on a 4 in major diameter, giving a metric-equivalent pitch of 3.175 mm. The size label is a nominal bore rather than a diameter.
Note. Diameters are quoted at the gauge plane. The thread is tapered 1:16 on diameter, so the actual size changes along the thread.
Thread profile
millimetres unless statedBasic information
- Designation
- 3-1/2-8 NPT
- Thread system
- National Pipe Taper
- Series
- pipe
- Standard
- ASME B1.20.1
- Basic profile
- Unified profile, 0.8P thread height
- Flank angle
- 60°
Dimensions
- Major diameter
- 4 in / 101.6 mm
- Pitch (metric equivalent)
- 3.175 mm
- Threads per inch
- 8
- Pitch diameter
- 3.9189 in / 99.54 mm
- Minor diameter (external)
- 3.8189 in / 97 mm
- Minor diameter (internal)
- 3.8646 in / 98.16 mm
- Thread depth
- 0.1 in / 2.54 mm
- Recommended tap drill
- Not applicable — see note
How these values are derived
tabulated standard valuesThis is a pipe thread, so the published dimensions are tabulated values from ASME B1.20.1 rather than figures derived from a formula: a pipe size is a nominal bore, and the relationship between that bore and the thread diameter is historical, not geometric. Diameters are quoted at the gauge plane; the thread itself is tapered, so the actual size changes along its length.
Compatible parts0 specification pages
No specification in this database uses 3-1/2-8 NPT taper. That is expected: the catalogue covers machine screws, bolts, nuts and washers, and pipe threads belong to fittings, which are a separate family of parts. The dimensions above are still the standard ones for the thread itself.
Related
Neighbouring sizes in National Pipe Taper.