This reference provides tap ground-thread limit (GH) recommendations and pitch diameter limits for Unified (UNC, UNF, UNS) and American National (NC, NF) screw threads in Class 2, Class 3, Class 2B, and Class 3B fits. The table covers nominal sizes from 0-80 through 1 1/2-12, listing the appropriate GH limit tap for each thread class and the corresponding minimum (basic) and maximum pitch diameter limits. These are general recommendations for average materials when taps are used with reasonable care.
Tap recommendations and pitch diameter limits by thread size and class
The table below lists recommended tap ground-thread limits and pitch diameter limits for each nominal thread size, threads per inch (TPI), and class of thread.
| Nominal Size | T.P.I. NC/UNC | T.P.I. NF/UNF | Class 2 | Class 3 | Class 2B | Class 3B | Min. All Classes (Basic) | Max. Class 2 | Max. Class 3 | Max. Class 2B | Max. Class 3B |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | , | 80 | GH1 | GH1 | GH2 | GH1 | .0519 | .0536 | .0532 | .0542 | .0536 |
| 1 | 64 | , | GH1 | GH1 | GH2 | GH1 | .0629 | .0648 | .0643 | .0655 | .0648 |
| 1 | , | 72 | GH1 | GH1 | GH2 | GH1 | .0640 | .0658 | .0653 | .0665 | .0659 |
| 2 | 56 | , | GH1 | GH1 | GH2 | GH1 | .0744 | .0764 | .0759 | .0772 | .0765 |
| 2 | , | 64 | GH1 | GH1 | GH2 | GH1 | .0759 | .0778 | .0773 | .0786 | .0779 |
| 3 | 48 | , | GH1 | GH1 | GH2 | GH1 | .0855 | .0877 | .0871 | .0885 | .0877 |
| 3 | , | 56 | GH1 | GH1 | GH2 | GH1 | .0874 | .0894 | .0889 | .0902 | .0895 |
| 4 | 40 | , | GH2 | GH1 | GH2 | GH2 | .0958 | .0982 | .0975 | .0991 | .0982 |
| 4 | , | 48 | GH1 | GH1 | GH2 | GH1 | .0985 | .1007 | .1001 | .1016 | .1008 |
| 5 | 40 | , | GH2 | GH1 | GH2 | GH2 | .1088 | .1112 | .1105 | .1121 | .1113 |
| 5 | , | 44 | GH1 | GH1 | GH2 | GH1 | .1102 | .1125 | .1118 | .1134 | .1126 |
| 6 | 32 | , | GH2 | GH1 | GH3 | GH2 | .1177 | .1204 | .1196 | .1214 | .1204 |
| 6 | , | 40 | GH2 | GH1 | GH2 | GH2 | .1218 | .1242 | .1235 | .1252 | .1243 |
| 8 | 32 | , | GH2 | GH1 | GH3 | GH2 | .1437 | .1464 | .1456 | .1475 | .1465 |
| 8 | , | 36 | GH2 | GH1 | GH2 | GH2 | .1460 | .1485 | .1478 | .1496 | .1487 |
| 10 | 24 | , | GH3 | GH1 | GH3 | GH3 | .1629 | .1662 | .1653 | .1672 | .1661 |
| 10 | , | 32 | GH2 | GH1 | GH3 | GH2 | .1697 | .1724 | .1716 | .1736 | .1726 |
| 12 | 24 | , | GH3 | GH1 | GH3 | GH3 | .1889 | .1922 | .1913 | .1933 | .1922 |
| 12 | , | 28 | GH3 | GH1 | GH3 | GH3 | .1928 | .1959 | .1950 | .1970 | .1959 |
| 1/4 | 20 | , | GH3 | GH2 | GH5 | GH3 | .2175 | .2211 | .2201 | .2224 | .2211 |
| 1/4 | , | 28 | GH3 | GH1 | GH4 | GH3 | .2268 | .2299 | .2290 | .2311 | .2300 |
| 5/16 | 18 | , | GH3 | GH2 | GH5 | GH3 | .2764 | .2805 | .2794 | .2817 | .2803 |
| 5/16 | , | 24 | GH3 | GH1 | GH4 | GH3 | .2854 | .2887 | .2878 | .2902 | .2890 |
| 3/8 | 16 | , | GH3 | GH2 | GH5 | GH3 | .3344 | .3389 | .3376 | .3401 | .3387 |
| 3/8 | , | 24 | GH3 | GH1 | GH4 | GH3 | .3479 | .3512 | .3503 | .3528 | .3516 |
| 7/16 | 14 | , | GH5 | GH3 | GH5 | GH3 | .3911 | .3960 | .3947 | .3972 | .3957 |
| 7/16 | , | 20 | GH3 | GH1 | GH5 | GH3 | .4050 | .4086 | .4076 | .4104 | .4091 |
| 1/2 | 13 | , | GH5 | GH3 | GH5 | GH3 | .4500 | .4552 | .4537 | .4565 | .4548 |
| 1/2 | , | 20 | GH3 | GH1 | GH5 | GH3 | .4675 | .4711 | .4701 | .4731 | .4717 |
| 9/16 | 12 | , | GH5 | GH3 | GH5 | GH3 | .5084 | .5140 | .5124 | .5152 | .5135 |
| 9/16 | , | 18 | GH3 | GH2 | GH5 | GH3 | .5264 | .5305 | .5294 | .5323 | .5308 |
| 5/8 | 11 | , | GH5 | GH3 | GH5 | GH3 | .5660 | .5719 | .5702 | .5732 | .5714 |
| 5/8 | , | 18 | GH3 | GH2 | GH5 | GH3 | .5889 | .5930 | .5919 | .5949 | .5934 |
| 3/4 | 10 | , | GH5 | GH3 | GH5 | GH5 | .6850 | .6914 | .6895 | .6927 | .6907 |
| 3/4 | , | 16 | GH3 | GH2 | GH5 | GH3 | .7094 | .7139 | .7126 | .7159 | .7143 |
| 7/8 | 9 | , | GH6 | GH4 | GH6 | GH4 | .8028 | .8098 | .8077 | .8110 | .8089 |
| 7/8 | , | 14 | GH4 | GH2 | GH6 | GH4 | .8286 | .8335 | .8322 | .8356 | .8339 |
| 1 | 8 | , | GH6 | GH4 | GH6 | GH4 | .9188 | .9264 | .9242 | .9276 | .9254 |
| 1 | , | 12 | GH4 | GH2 | GH6 | GH4 | .9459 | .9515 | .9499 | .9535 | .9516 |
| 1 | , | 14* | GH4 | GH2 | GH6 | GH4 | .9536 | .9585 | .9572 | .9609 | .9590 |
| 1 1/8 | 7 | , | GH8 | GH4 | GH8 | GH4 | 1.0322 | 1.0407 | 1.0381 | 1.0416 | 1.0393 |
| 1 1/8 | , | 12 | GH4 | GH4 | GH6 | GH4 | 1.0709 | 1.0765 | 1.0749 | 1.0787 | 1.0768 |
| 1 1/4 | 7 | , | GH8 | GH4 | GH8 | GH4 | 1.1572 | 1.1657 | 1.1631 | 1.1668 | 1.1644 |
| 1 1/4 | , | 12 | GH4 | GH4 | GH6 | GH4 | 1.1959 | 1.2015 | 1.1999 | 1.2039 | 1.2019 |
| 1 3/8 | 6 | , | GH8 | GH4 | GH8 | GH4 | 1.2667 | 1.2768 | 1.2738 | 1.2771 | 1.2745 |
| 1 3/8 | , | 12 | GH4 | GH4 | GH6 | GH4 | 1.3209 | 1.3265 | 1.3249 | 1.3291 | 1.3270 |
| 1 1/2 | 6 | , | GH8 | GH4 | GH8 | GH4 | 1.3917 | 1.4018 | 1.3988 | 1.4022 | 1.3996 |
| 1 1/2 | , | 12 | GH4 | GH4 | GH6 | GH4 | 1.4459 | 1.4515 | 1.4499 | 1.4542 | 1.4522 |
*UNS
What the thread class designations mean
Class 2 and Class 2B: General-purpose external (Class 2) and internal (Class 2B) thread fits with standard allowances and tolerances.
Class 3 and Class 3B: Closer-tolerance external (Class 3) and internal (Class 3B) thread fits with tighter pitch diameter limits than Class 2 threads.
What the GH limit designations mean
GH limits (GH1, GH2, GH3, GH4, GH5, GH6, GH8) specify the ground-thread tolerance range for taps. Lower GH numbers indicate tighter tolerances, higher numbers indicate looser tolerances. The recommended GH limit varies by thread class and size to produce the specified pitch diameter range in the tapped hole.
How to use this reference
- Identify the nominal thread size and threads per inch (TPI) for the desired thread.
- Determine whether the thread is coarse (NC/UNC) or fine (NF/UNF).
- Select the thread class (Class 2, 3, 2B, or 3B) required by the application.
- Read across the row to find the recommended GH limit tap for that class.
- Verify that the resulting pitch diameter will fall within the minimum (basic) and maximum limits shown for that class.
Application notes
These recommendations apply to average materials when taps are used with reasonable care. If the specified tap does not produce a satisfactory gauge fit in the work, consult the factory for alternative recommendations.
Source and updates
Source: taylortool.com engineering data. Reviewed September 2026.