USCTI Table 302-A specifies dimensions for ground thread taps with optional neck and optional shortened thread length, covering machine screw sizes #4 through 1 inch and metric sizes M3 through M25. The thread length is based on 12 pitches of the UNC thread series and is a minimum value with no tolerance. When thread length and neck length are added together, the total must be no less than the minimum thread length specified in standard Table 302.
What tap sizes are covered by Table 302-A?
Table 302-A covers machine screw taps from size #4 (0.1120 inch) through 1 inch (1.0000 inch) and metric taps from M3 (0.1181 inch) through M25 (0.9843 inch). The table provides dimensions for three blank designs depending on the nominal diameter range.
What are the general dimensions for optional neck taps?
The following table lists all dimensions for ground thread taps with optional neck and shortened thread length.
| Nominal Diameter Range (Inches) | Mach. Screw Size No. | Nominal Fractional Diameter (Inches) | Nominal Metric Diameter Millimeters (Inch) | Blank Design No. | Overall Length L | Thread Length l | Neck Length l1 | Square Length l2 | Shank Diameter d1 | Size of Square a |
|---|---|---|---|---|---|---|---|---|---|---|
| .104 to .117 | 4 | (.1120) | – | 1 | 1.88 | .31 | 0.25 | 0.19 | 1.141 | 0.11 |
| .117 to .130 | 5 | (.1250) | M3 (.1181) | 1 | 1.94 | .31 | 0.31 | 0.19 | 1.141 | 0.11 |
| .130 to .145 | 6 | (.1380) | M3.5 (.1378) | 1 | 2 | 0.38 | 0.31 | 0.19 | 1.141 | 0.11 |
| .145 to .171 | 8 | (.1640) | M4 (.1575) | 1 | 2.13 | 0.38 | 0.38 | 0.25 | 0.168 | 0.131 |
| .171 to .197 | 10 | (.1900) | M4.5(.1772), M5(.1969) | 1 | 2.38 | .50 | 0.38 | 0.25 | 0.194 | 0.152 |
| .197 to .223 | 12 | (.2160) | – | 1 | 2.38 | .50 | 0.44 | 0.28 | 0.22 | 0.165 |
| .223 to .260 | – | 1/4 (.2500) | M6 (.2362) | 2 | 2.5 | 0.63 | 0.38 | 0.31 | 0.255 | 0.191 |
| .260 to .323 | – | 5/16 (.3125) | M7 (.2756), M8(.3150) | 2 | 2.72 | 0.69 | 0.44 | 0.38 | 0.318 | 0.238 |
| .323 to .395 | – | 3/8 (.3750) | M10 (.3937) | 2 | 2.94 | 0.75 | 0.50 | 0.44 | 0.381 | 0.286 |
| .395 to .448 | – | 7/16 (.4375) | – | 3 | 3.16 | .88 | – | 0.41 | 0.323 | 0.242 |
| .448 to .510 | – | 1/2 (.5000) | M12 (.4724) | 3 | 3.38 | .94 | – | 0.44 | 0.367 | 0.275 |
| .510 to .573 | – | 9/16 (.9375) | M14 (.5512) | 3 | 3.59 | 1.00 | – | 0.5 | 0.429 | 0.322 |
| .573 to .635 | – | 5/8 (0.6250) | M16 (0.6299) | 3 | 3.81 | 1.09 | – | 0.56 | 0.48 | 0.36 |
| Over 0.635 To 0.709 | – | 11/16 (0.6875) | M18 (0.7087) | 3 | 4.03 | 1.09 | – | 0.63 | 0.542 | 0.406 |
| Over 0.709 To 0.760 | – | 3/4 (0.7500) | – | 3 | 4.25 | 1.22 | – | 0.69 | 0.59 | 0.442 |
| Over 0.760 To 0.823 | – | 13/16 (0.8125) | M20 (0.7874) | 3 | 4.47 | 1.22 | – | 0.69 | 0.652 | 0.489 |
| Over 0.823 To 0.885 | – | 7/8 (0.8750) | M22 (0.8661) | 3 | 4.69 | 1.34 | – | 0.75 | 0.697 | 0.523 |
| Over 0.885 To 0.948 | – | 15/16 (0.9375) | M24 (0.9449) | 3 | 4.91 | 1.34 | – | 0.75 | 0.760 | 0.570 |
| Over 0.948 To 1.010 | – | 1 (1.0000) | M25 (0.9843) | 3 | 5.13 | 1.50 | – | 0.81 | 0.800 | 0.600 |
How is thread length determined for optional neck taps?
Thread length (l) is based on a length of 12 pitches of the UNC thread series. Thread length is a minimum value and has no tolerance. When thread length (l) is added to neck length (l1), the total shall be no less than the minimum Table 302 thread length.
What blank designs are used?
Three blank designs are specified: Blank Design 1 is used for sizes #4 through #12, Blank Design 2 is used for sizes 1/4 inch through 3/8 inch, and Blank Design 3 is used for sizes 7/16 inch through 1 inch. Blank Designs 1 and 2 include a neck length dimension (l1), while Blank Design 3 does not specify a neck length.
What tolerances apply to these dimensions?
Unless otherwise specified, all tolerances are in accordance with Table 302. For eccentricity tolerances, see Table 317. Thread length (l) is a minimum value and has no tolerance.
Source and updates
Source: taylortool.com engineering data. Reviewed September 2026.