How to determine the correct core hole size when threading
What is the core hole size?
The core hole is the hole before the thread is cut.
If it's too small, the drill bit will break. If it's too large, the thread won't engage.
Rule of thumb
For metric ISO threads:
Core hole = nominal diameter – pitch
Example:
-
M6 (pitch 1.0 mm) → 5.0 mm
-
M8 (pitch 1.25 mm) → 6.8 mm
Core hole dimension tables are mandatory in everyday workshop life.
A correct core hole size is the basis for every thread.
If you work precisely here, you will save yourself trouble and rework.
Core hole size table – metric ISO threads
| thread | Pitch (mm) | Core hole size (mm) | Notice |
|---|---|---|---|
| M3 | 0.5 | 2.5 | Standard size |
| M4 | 0.7 | 3.3 | Universal |
| M5 | 0.8 | 4.2 | Mechanical engineering |
| M6 | 1.0 | 5.0 | standard |
| M8 | 1.25 | 6.8 | All-round |
| M10 | 1.5 | 8.5 | Common workshop measurement |
| M12 | 1.75 | 10.2 | Plant engineering |
| M16 | 2.0 | 14.0 | coarse thread |
| M20 | 2.5 | 17.5 | industry |
Metric fine threads (DIN 13-2)
| thread | Pitch (mm) | Core hole size (mm) |
|---|---|---|
| M6 × 0.75 | 0.75 | 5.25 |
| M8 × 1.0 | 1.0 | 7.0 |
| M10 × 1.0 | 1.0 | 9.0 |
| M12 × 1.25 | 1.25 | 10.8 |
| M16 × 1.5 | 1.5 | 14.5 |
| M20 × 1.5 | 1.5 | 18.5 |
UNC – Unified Coarse Thread
| thread | Thread pitch (TPI) | Core hole size (mm) | Notice |
|---|---|---|---|
| 1/4-20 UNC | 20 | 5.1 | Standard UNC |
| 5/16-18 UNC | 18 | 6.7 | Mechanical engineering |
| 3/8-16 UNC | 16 | 8.5 | General construction |
| 1/2-13 UNC | 13 | 10.8 | Heavy construction |
| 3/4-10 UNC | 10 | 16.4 | Large thread |
UNF – Unified Fine Thread
| thread | Thread pitch (TPI) | Core hole size (mm) |
|---|---|---|
| 1/4-28 UNF | 28 | 5.3 |
| 5/16-24 UNF | 24 | 6.8 |
| 3/8-24 UNF | 24 | 8.6 |
| 1/2-20 UNF | 20 | 11.0 |
| 3/4-16 UNF | 16 | 16.6 |
NPT – Tapered pipe threads
| thread | Gradient (TPI) | Core hole size (mm) | Application |
|---|---|---|---|
| 1/8-27 NPT | 27 | 8.6 | Hydraulics, air |
| 1/4-18 NPT | 18 | 11.5 | Air, gas, lubrication |
| 3/8-18 NPT | 18 | 14.75 | Hydraulic systems |
| 1/2-14 NPT | 14 | 18.6 | water pipes |
| 3/4-14 NPT | 14 | 23.6 | industrial plants |
💡 Practical tip: For metric threads, the rule of thumb is core hole = nominal diameter − pitch . For UNC, UNF, and NPT threads, please use the table values, as the profile and taper depth are taken into account. Use cutting oil and a short countersink for clean cuts and longer tool life.