3-1/2-8 NPT Thread Dimensions and Tap Drill
3-1/2 NPT, written in full as 3-1/2-8 NPT, is the American taper pipe thread for nominal 3-1/2 inch pipe: 8 threads per inch cut on a 1 in 16 taper, on pipe with an outside diameter of 4.000 in (101.60 mm). The nominal size names the pipe and is not a dimension you can measure on the thread.
- Tap drill3-3/4"3.7500 in, 95.25 mm
- Threads per inch8pitch 3.175 mm
- Pipe outside diameter4.000 in101.60 mm
- Pitch Ø at hand-tight plane3.88881 in98.776 mm
- Hand-tight engagement0.821 in6.57 threads
- Effective thread length1.2500 in10.00 threads
Where the diameters are measured
A taper thread has no single pitch diameter. ASME B1.20.1 defines it at the end of the pipe (E0), at the plane where a fitting stops when screwed on by hand (E1), and at the end of the effective thread (E2). Each step of 1 in along the axis adds 1/16 in to the diameter.
3-1/2-8 NPT dimensions
| Dimension | in | mm |
|---|---|---|
| Pipe outside diameter D | 4.0000 | 101.600 |
| Pitch P | 0.12500 | 3.175 |
| Pitch diameter at pipe end E0 | 3.83750 | 97.472 |
| Pitch diameter at hand-tight plane E1 | 3.88881 | 98.776 |
| Pitch diameter at end of effective thread E2 | 3.91562 | 99.457 |
| Hand-tight engagement L1 | 0.8210 | 20.853 |
| Effective thread length L2 | 1.2500 | 31.750 |
| Minor diameter at pipe end K0 | 3.7375 | 94.932 |
| Thread height h (0.8 × P) | 0.10000 | 2.540 |
| Property | Value |
|---|---|
| Threads per inch | 8 |
| Flank angle | 60° (30° each side) |
| Taper | 1 in 16 on diameter (3/4 in per foot) |
| Taper angle to the axis | 1° 47′ (1.7899°) |
| Threads engaged by hand | 6.57 |
| Wrench make-up | 2 threads |
Basic dimensions from ASME B1.20.1 Table 2. Millimetre values are conversions.
3-1/2 NPT tap drill size
Tap drill for 3-1/2 NPT: 3-3/4" (3.7500 in, 95.25 mm)
From the pipe tap drill table in Machinery’s Handbook. ASME B1.20.1 suggests drills only up to the 2-1/2 size.
| Method | Drill | in | mm | Against K0 (in) |
|---|---|---|---|---|
| Standard tap drill chartusual choice | 3-3/4" | 3.7500 | 95.25 | +0.0125 |
The hole for a taper tap is a compromise, because a straight drilled hole meets a tapered thread. The minor diameter of the thread is 3.7375 in at the small end (K0) and grows by 1/16 in per inch of depth.
ASME B1.20.1 notes that its suggested drills do not guarantee fully formed threads over the whole hand-tight length.
How far 3-1/2 NPT screws in
A 3-1/2 NPT fitting turns in by hand for 0.821 in (20.9 mm), about 6.6 threads, before the flanks meet. The standard then allows 2 more threads of wrench make-up, for a total engagement of about 1.071 in (27.2 mm). Tolerance on the hand-tight position is one thread either way, so a joint that stops a turn early or late is still within the standard.
NPT threads do not seal by themselves. The crests and roots leave a spiral gap, so ASME B1.20.1 calls for a sealant (PTFE tape or pipe compound) where the joint must be pressure-tight. Dryseal NPTF threads to ASME B1.20.3 have the same pitch and taper but controlled crests and roots that seal without it.
3-1/2 NPT against G3-1/2 BSP
The British Standard Pipe thread of the same nominal size has a different pitch (11 against 8 threads per inch), a 55° flank angle instead of 60° and a major diameter of 100.330 mm against 101.600 mm. The two do not screw together properly. Use an adapter.
| Thread | TPI | Major Ø (mm) | Major Ø (in) | Flank angle | Taper |
|---|---|---|---|---|---|
| 3-1/2-8 NPT (this thread) | 8 | 101.600 | 4.0000 | 60° | 1 in 16 |
| G3-1/2 (BSPP) | 11 | 100.330 | 3.9500 | 55° | parallel |
Related sizes and tools
Sources: ASME B1.20.1 (dimensions, Table 2; suggested drills, Appendix), Machinery’s Handbook and tap makers’ charts (shop tap drill sizes). How this site checks its data. Last reviewed 29 September 2026.