UNF · cutting tap · nominal dimensions
3/8-24 UNF
tap drill size.
For 3/8-24 UNF, the reference drill is Q: 0.3320 in (8.433 mm).
The reference value is kept separate from the 70% and 75% calculated targets. Actual fit still depends on thread class, tool, material, runout, and the hole you really make.
Detail A · basic 60° profile
What the percentage describes.
“Theoretical full thread %” is radial thread height from nominal geometry. It is not axial engagement length and not a fit-class acceptance measurement.
Real drill alternatives
Neighboring sizes.
These are diameter neighbors, not silent recommendations. A positive delta makes a larger hole and a lower nominal theoretical percentage; a negative delta does the opposite.
| Bit | System | Diameter | Delta | Calculated full thread |
|---|---|---|---|---|
| 8.4 mm | metric | 0.3307 in / 8.4 mm | -0.0013 in | 81.8% |
| 8.5 mm | metric | 0.3346 in / 8.5 mm | +0.0026 in | 74.6% |
| 21/64 | fractional | 21/64 in / 8.334 mm | -0.0039 in | 86.6% |
| 8.3 mm | metric | 0.3268 in / 8.3 mm | -0.0052 in | 89.1% |
| 8.6 mm | metric | 0.3386 in / 8.6 mm | +0.0066 in | 67.3% |
| R | letter | 0.3390 in / 8.611 mm | +0.0070 in | 66.5% |
Why 3D is useful here
A thread is a helix, not a row of triangles.
Load a draggable cutaway to see the continuous internal thread. Exact diameter and profile comparisons remain in the 2D drawing above.
Evidence and limits
Why this row says provisional.
Tap-drill row: A familiar chart or D-minus-pitch value that was not closed against a single row in the selected OSG/Guhring public tables. Kept visible for audit, never labeled normative or manufacturer-published.
Open Drill Bit Size Chart source
60° geometry: Public primary reference for Unified and metric 60-degree thread geometry. The percentage-thread equation is a theoretical basic-profile calculation, not a fit or torque guarantee.
Open National Institute of Standards and Technology geometry source
The repository also records 11 source records and a dataset version on every page.