USCTI Table 338 specifies tap thread limits, projection dimensions, taper per foot limits, L1 lengths and recommended tap drill sizes for ground thread taper pipe taps in three thread forms: American National Standard Taper Pipe Thread (NPT), Aeronautical National Taper Pipe Thread (ANPT), and Dryseal American National Standard Taper Pipe Thread (NPTF). The table covers nominal sizes from 1/16 inch through 4 inches with threads per inch ranging from 27 to 8.
What thread forms are covered by this standard?
This standard covers three taper pipe thread forms:
- American National Standard Taper Pipe Thread Form (NPT)
- Aeronautical National Taper Pipe Thread Form (ANPT)
- Dryseal American National Standard Taper Pipe Thread Form (NPTF)
All three forms use the same tap thread limits, projection tolerances, taper per foot limits, L1 lengths and tap drill sizes.
Tap thread limits and reference dimensions
The following table lists tap thread limits and reference dimensions for ground thread taper pipe taps.
| Normal Size Inches | Threads Per Inch | Projection * Inches | Projection Tolerance + or – | Taper Per Foot Limits Minimum | Taper Per Foot Limits Maximum | L1 Length | Tap Drill Size** NPT, ANPT, NPTF |
|---|---|---|---|---|---|---|---|
| 1/16 | 27 | .312 | .063 | .719 | .781 | .160 | C |
| 1/8 | 27 | .063 | .063 | .719 | .781 | .1615 | Q |
| 1/4 | 18 | .459 | .063 | .719 | .781 | .2278 | 7/16 |
| 3/8 | 18 | .454 | .063 | .719 | .781 | .240 | 9/16 |
| 1/2 | 14 | .579 | .063 | .719 | .781 | .320 | 45/64 |
| 3/4 | 14 | .565 | .063 | .719 | .781 | .339 | 29/32 |
| 1″ | 11 – 1/2 | .678 | .94 | .719 | .781 | .400 | 1-9/64 |
| 1 – 1/4 | 11 – 1/2 | .686 | .94 | .719 | .781 | .420 | 1-31/64 |
| 1 – 1/2 | 11 – 1/2 | .699 | .94 | .719 | .781 | .420 | 1-23/32 |
| 2″ | 11 – 1/2 | .667 | .94 | .719 | .781 | .436 | 2-3/16 |
| 2 – 1/2 | 8 | .925 | .94 | .734 | .781 | .682 | 2-39/64 |
| 3″ | 8 | .925 | .94 | .734 | .781 | .766 | 3-15/64 |
| 3 – 1/2 | 8 | .938 | .125 | .734 | .781 | .821 | ……. |
| 4 | 8 | .950 | .125 | .734 | .781 | .844 | ……. |
What does projection mean in this table?
Projection is the distance the small end of the tap projects through the L1 Taper Thread Ring Gage. This dimension is measured in inches and has a tolerance of plus or minus the value shown in the Projection Tolerance column.
What tap drill sizes are recommended?
The recommended tap drill sizes permit direct tapping without reaming the hole, but only give a full thread for approximately the L1 length. The drill sizes range from letter size C for 1/16 inch taps to fractional sizes up to 3-15/64 inches for 3 inch taps.
What is the lead tolerance for taper pipe threads?
A maximum lead deviation of plus or minus .0005 inches within any two threads not farther than 1 inch is permitted.
Source and updates
Source: taylortool.com engineering data. Reviewed September 2026.