Use a 5.0 mm drill for an M6 tap; for a clearance hole use about 6.5–7.0 mm (1/4" is 6.35 mm).
If you want clear, practical advice on what drill size for M6 bolt, you’re in the right place. I’ve worked with metric bolts in metal and wood projects for years, and this guide explains the difference between tap drills, clearance drills, and pilot holes. Read on to learn exact sizes, how to calculate drill bits for different thread pitches, real-world tips, and common mistakes to avoid when drilling for an M6 bolt.
Understanding M6 bolt and thread basics
An M6 bolt has a nominal major diameter of 6.0 mm. The most common M6 thread pitch is 1.0 mm (called coarse). There are also fine pitches such as 0.75 mm and 0.5 mm for specialized uses.
Why this matters: the drill size you pick depends on whether you want to tap threads into the hole or let the bolt pass freely. Asking “what drill size for M6 bolt” can mean three different things: tap drill, clearance hole, or pilot hole. Choose the right one to avoid weak threads or a loose bolt.
Drill sizes by purpose: tap, clearance, and pilot
- Tap drill (create internal threads)
- Standard M6x1.0: use a 5.0 mm drill bit. This is the common rule: tap drill = major diameter – pitch.
- M6x0.75 (fine): use 5.25 mm.
- M6x0.5 (extra fine): use 5.5 mm.
- Clearance hole (bolt passes through)
- Close running fit: 6.2–6.4 mm. Use when minimal play is needed.
- Standard fit: 6.5–7.0 mm. Most projects use this range.
- Loose fit or alignment: 7.0–8.0 mm. Use for easier assembly or misalignment tolerance.
- Common inch substitute: 1/4" (6.35 mm) works well for many M6 bolts.
- Pilot hole (for countersinks or self-tapping screws)
- For countersunk M6 bolts, pilot depends on countersink angle and thread type.
- For self-tapping or sheet-metal fasteners, follow manufacturer guidance.
If you search “what drill size for M6 bolt,” decide first if you’re tapping threads or making a clearance hole. That decision sets the size.
How to calculate tap drill size for M6
- Use the simple formula: Tap drill = Major diameter – Thread pitch.
- Example: M6 × 1.0 → 6.0 – 1.0 = 5.0 mm.
- Example: M6 × 0.75 → 6.0 – 0.75 = 5.25 mm.
- For metric threads, this formula gives a good start. If you want a specific thread engagement percentage, adjust slightly.
- 75% thread engagement is common in structural work. Use manufacturers’ charts for exact values.
- When in doubt, choose the published tap drill for the thread pitch. This avoids undercut or weak threads.
Understanding the math answers “what drill size for M6 bolt” precisely when you need to tap threads.
Practical tips, tools, and common mistakes
- Tools to use
- High-speed steel (HSS) twist bits for general steel and wood.
- Cobalt or coated bits for hard or stainless steels.
- A drill press gives straight holes and better control than a hand drill.
- Drilling tips
- Use cutting fluid for metals to extend bit life and get cleaner threads.
- Start with a center punch to stop the bit from wandering.
- Back out chips frequently when drilling deep holes.
- Common mistakes to avoid
- Using the wrong drill for tapped threads—too large and threads are weak, too small and taps will bind.
- Skipping lubrication when drilling steel—this shortens bit life and ruins holes.
- Assuming 1/4" is perfect — it’s close (6.35 mm) but may be slightly loose or tight depending on bolt tolerance.
- Personal experience
- I once used a 6.0 mm drill for a clearance hole thinking it would fit. The bolt wouldn’t pass easily. I re-drilled to 6.5 mm and all was fine. Now I always check tight vs. running fit before drilling.
- On stainless, I always choose cobalt bits and slow feed. That saved me from broken bits several times.
These tips will help you pick the correct drill when asking “what drill size for M6 bolt” on your next job.
Quick PAA-style questions (People also ask)
What drill size for M6 bolt when tapping threads?
For standard M6×1.0 tapping, use a 5.0 mm drill. For fine pitches, subtract the pitch from 6.0 mm (for example, 5.25 mm for 0.75 pitch).
What drill size for M6 bolt as a clearance hole?
Use 6.5–7.0 mm for a typical clearance hole. For close fits, aim 6.2–6.4 mm; for loose fits, 7.0–8.0 mm works well.
Can I use a 1/4" drill for an M6 bolt?
Yes. 1/4" equals 6.35 mm and is commonly used as a clearance drill for M6 bolts. It’s a practical choice for many projects.
Frequently Asked Questions of what drill size for m6 bolt
What drill size for M6 bolt if the thread pitch is unknown?
If you don’t know the pitch, measure the bolt threads or use a thread gauge. For standard M6 with unknown pitch, assume 1.0 mm and use a 5.0 mm tap drill for tapping.
Is a 6 mm drill OK for an M6 bolt clearance hole?
A 6 mm drill is often too tight for clearance. Expect a snug fit; you may need to ream or enlarge to 6.5–7.0 mm for smooth assembly.
How deep should I drill for an M6 tapped hole?
Drill slightly deeper than the fastener’s thread length, usually 1.5× the bolt diameter for through-tapped holes. For blind holes, drill 1–2 mm deeper than the tapping depth to collect chips.
Can I drill M6 holes in aluminum using the same sizes?
Yes. Use the same tap and clearance sizes for aluminum. Use light cutting fluid or oil and lower drill speeds for a clean hole.
What drill size for M6 bolt in thin sheet metal?
For thin sheet metal where you will use a nut and bolt, use 6.5–7.0 mm for clearance. If using self-tapping screws, follow the screw maker’s pilot size instead.
Conclusion
Knowing what drill size for M6 bolt depends on one question: are you tapping threads or making a clearance hole? Use 5.0 mm for standard M6×1.0 tap drills, 5.25 mm for M6×0.75, and 6.5–7.0 mm for typical clearance holes (1/4" is a handy substitute). Choose tools and speeds for the material, and always test on scrap if you’re unsure. Try these sizes on a practice piece, then apply them confidently to your project. If this helped, leave a comment with your project details or subscribe for more practical fastening and drilling tips.