8-8 NPT Thread Dimensions and Tap Drill
8 NPT, written in full as 8-8 NPT, is the American taper pipe thread for nominal 8 inch pipe: 8 threads per inch cut on a 1 in 16 taper, on pipe with an outside diameter of 8.625 in (219.07 mm). The nominal size names the pipe and is not a dimension you can measure on the thread.
- Bore before tapping8.3336 in211.67 mm
- Threads per inch8pitch 3.175 mm
- Pipe outside diameter8.625 in219.07 mm
- Pitch Ø at hand-tight plane8.50003 in215.901 mm
- Hand-tight engagement1.063 in8.50 threads
- Effective thread length1.7125 in13.70 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.
8-8 NPT dimensions
| Dimension | in | mm |
|---|---|---|
| Pipe outside diameter D | 8.6250 | 219.075 |
| Pitch P | 0.12500 | 3.175 |
| Pitch diameter at pipe end E0 | 8.43359 | 214.213 |
| Pitch diameter at hand-tight plane E1 | 8.50003 | 215.901 |
| Pitch diameter at end of effective thread E2 | 8.54062 | 216.932 |
| Hand-tight engagement L1 | 1.0630 | 27.000 |
| Effective thread length L2 | 1.7125 | 43.497 |
| Minor diameter at pipe end K0 | 8.3336 | 211.673 |
| 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 | 8.50 |
| Wrench make-up | 2 threads |
Basic dimensions from ASME B1.20.1 Table 2. Millimetre values are conversions.
8 NPT tap drill size
Bore before tapping 8 NPT: 8.3336 in (211.67 mm)
This is K0, the basic minor diameter at the small end of the thread.
No tap drill is published for 8 NPT. Threads this large are bored or milled, and ASME B1.20.1 gives the minor diameter K0 as the basis for the hole size. Taper the bore at 1 in 16 where full threads are needed over the whole engagement.
How far 8 NPT screws in
A 8 NPT fitting turns in by hand for 1.063 in (27.0 mm), about 8.5 threads, before the flanks meet. The standard then allows 2 more threads of wrench make-up, for a total engagement of about 1.313 in (33.4 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.
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.