Metric coarse · cutting tap · nominal dimensions
M8 × 1.25
tap drill size.
For M8 × 1.25, the reference drill is 6.8 mm: 0.2677 in (6.800 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 |
|---|---|---|---|---|
| H | letter | 0.2660 in / 6.756 mm | -0.0017 in | 76.6% |
| 17/64 | fractional | 17/64 in / 6.747 mm | -0.0021 in | 77.2% |
| 6.7 mm | metric | 0.2638 in / 6.7 mm | -0.0039 in | 80.1% |
| 6.9 mm | metric | 0.2717 in / 6.9 mm | +0.0039 in | 67.7% |
| I | letter | 0.2720 in / 6.909 mm | +0.0043 in | 67.2% |
| G | letter | 0.2610 in / 6.629 mm | -0.0067 in | 84.4% |
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.