3-1/2" NPT Thread Specifications
Complete dimensions for 3-1/2" NPT (National Pipe Taper) thread per ASME B1.20.1. TPI: 8, Pitch: 0.12500" (3.175 mm). Taper: 1 in 16 on diameter (3/4" per foot).
Thread Dimensions
| Designation | 3-1/2" NPT |
| Nominal Pipe Size | 3-1/2" |
| TPI | 8 |
| Pitch | 0.12500" (3.175 mm) |
| Outside Diameter | 4.0000" (101.60 mm) |
| Pitch Diameter at E0 (hand-tight) | 3.83750" (97.472 mm) |
| Pitch Diameter at E1 (wrench makeup) | 3.88750" (98.742 mm) |
| Minor Diameter at E0 | 3.74097" (95.021 mm) |
| Taper Angle | 1.7899° (1:16 taper ratio, 3/4"/ft) |
| Thread Depth (h = 0.8p) | 0.10000" (2.540 mm) |
Values per ASME B1.20.1 Table 1. E0 = hand-tight engagement pitch diameter; E1 = wrench-makeup pitch diameter.
Tap Drill Size
Tap drill: 3-3/4" (3.7500" / 95.25 mm)
NPT tap drills allow the tap to cut the taper thread. The tap itself creates the taper geometry; drill size provides initial clearance at the large end.
Common Applications
- Plumbing connections
- Gas piping
- Hydraulic fittings
- Pneumatic connections