Pick your screw, get your drill size. Nothing else.
What screw or bolt are you tapping for?
Close Fit Clearance Hole
8.4
mm
💡 Drill first, then tap. Add cutting oil or WD-40.
Need RPM, materials & form taps? →
DRILL THIS SIZE HOLE
6.8
mm
Then tap your M8 × 1.25 thread
THREAD CROSS-SECTION
Adjust engagement % & material →
Common Tap Drill Reference Table
Standard sizes at 75% engagement.
| Thread | Major Dia | Pitch/TPI | Tap Drill (75%) | Exact Dia | Close Fit | Free Fit |
|---|
Frequently Asked Questions
What is a tap drill size and how is it calculated?
The tap drill is the hole you drill before tapping. Formula: Tap Drill
= Major Dia − (1.08253 × Pitch × 0.75). For M8×1.25 → 6.78 mm, rounded to 6.8 mm standard drill.
What thread engagement percentage should I use?
75% is industry standard. Use 65% for aluminum, 60–65% for stainless to
reduce tap breakage risk.
Cut tap vs form (roll) tap — what's the difference?
A cut tap removes chips. A form tap displaces material
— stronger threads, no chips, larger pilot hole needed. Never use on brittle materials.
What drill for M6, M8, M10, M12?
M6×1.0 → 5.0 mm, M8×1.25 → 6.8 mm, M10×1.5 →
8.5 mm, M12×1.75 → 10.2 mm.
What drill for 1/4-20, 5/16-18, 3/8-16?
1/4-20 → #7 (0.201"), 5/16-18 → F (0.257"),
3/8-16 → 5/16" (0.3125").
How fast should I run the tap (RPM)?
RPM = (SFM × 3.82) ÷ Diameter (in). Aluminum: ~100 SFM. Steel: ~50 SFM. Stainless: ~20
SFM. Always go slow — broken taps are expensive.