Tapping Drill Sizes.
Before tapping a hole with internal threads, you must drill a pilot hole using a specific **tap drill size**. Drilling the hole too small will break your tap, while drilling it too large will result in weak, stripped threads. Use the guide below to select the right drill bit.
Tap Drill vs. Clearance Drill
- Tap Drill: The drill bit size used to make the initial hole that is *slightly smaller* than the bolt diameter. The threads will be cut into this material, creating ~75% thread depth.
- Clearance Drill: The drill bit size used in the mating piece of material. It is *slightly larger* than the bolt major diameter so the screw can slide through without binding.
The Metric Tapping Rule of Thumb
For metric standard coarse threads, you can easily calculate the required tap drill diameter using a simple formula:
Example: For an M6 × 1.0 thread: 6.0mm - 1.0mm = 5.0mm drill bit.
Metric Tapping & Clearance Chart (ISO)
All sizes in millimeters (mm)| Thread Size | Pitch (Coarse) | Tap Drill (mm) | Clearance Close Fit | Clearance Free Fit |
|---|---|---|---|---|
| M2 | 0.40 mm | 1.60 mm | 2.10 mm | 2.20 mm |
| M2.5 | 0.45 mm | 2.05 mm | 2.60 mm | 2.70 mm |
| M3 | 0.50 mm | 2.50 mm | 3.20 mm | 3.40 mm |
| M4 | 0.70 mm | 3.30 mm | 4.30 mm | 4.50 mm |
| M5 | 0.80 mm | 4.20 mm | 5.30 mm | 5.50 mm |
| M6 | 1.00 mm | 5.00 mm | 6.40 mm | 6.60 mm |
| M8 | 1.25 mm | 6.80 mm | 8.40 mm | 9.00 mm |
| M10 | 1.50 mm | 8.50 mm | 10.50 mm | 11.00 mm |
| M12 | 1.75 mm | 10.20 mm | 13.00 mm | 13.50 mm |
| M14 | 2.00 mm | 12.00 mm | 15.00 mm | 15.50 mm |
| M16 | 2.00 mm | 14.00 mm | 17.00 mm | 17.50 mm |
Imperial Tapping & Clearance Chart (UTS)
Standard UNC & UNF Sizes| Thread Size | TPI | Tap Drill (Standard) | Tap Drill (Decimal/mm) | Clearance Close |
|---|---|---|---|---|
| #4-40 | 40 UNC | #43 | 0.0890" (2.26mm) | #32 (2.95mm) |
| #6-32 | 32 UNC | #36 | 0.1065" (2.71mm) | #27 (3.66mm) |
| #8-32 | 32 UNC | #29 | 0.1360" (3.45mm) | #18 (4.31mm) |
| #10-24 | 24 UNC | #25 | 0.1495" (3.80mm) | #9 (4.98mm) |
| #10-32 | 32 UNF | #21 | 0.1590" (4.04mm) | #9 (4.98mm) |
| 1/4"-20 | 20 UNC | #7 | 0.2010" (5.11mm) | F (6.53mm) |
| 1/4"-28 | 28 UNF | #3 | 0.2130" (5.41mm) | F (6.53mm) |
| 5/16"-18 | 18 UNC | F | 0.2570" (6.53mm) | P (8.20mm) |
| 5/16"-24 | 24 UNF | I | 0.2720" (6.91mm) | P (8.20mm) |
| 3/8"-16 | 16 UNC | 5/16" | 0.3125" (7.94mm) | W (9.80mm) |
| 3/8"-24 | 24 UNF | Q | 0.3320" (8.43mm) | W (9.80mm) |
Best Practices for Tapping Threads
1. Use Cutting Lubricant
Never tap dry. Always use appropriate cutting fluid or oil (e.g., WD-40 for aluminum, dark cutting oil for steel, soapy water or dry for cast iron) to reduce friction and heat.
2. Reverse Regularly
When tapping by hand, turn the tap forward 1/2 to 1 full turn, then reverse it 1/4 to 1/2 turn. This breaks the metal chips and prevents them from binding or breaking the tap.
3. Keep It Perpendicular
Ensure your pilot hole is straight and the tap starts perfectly square to the material surface. A crooked tap will result in misaligned threads or a broken tool.