What Size Drill Bit for an M6 Screw? (Tap and Clearance Hole Guide)

Introduction: Getting the Right Hole for Your M6 Screw

If you've ever found yourself staring at a drill bit set wondering, "What size drill bit for an M6 screw?" you're not alone. This is one of the most common questions in DIY projects and professional workshops alike. Getting the hole size right is crucial—too small and you'll struggle or break your tap; too large and your threads won't hold properly. In this comprehensive guide, we'll demystify the process and give you the exact dimensions you need for both tapping and clearance holes for M6 screws.

What is M6? Understanding Metric Screw Sizing

Before we dive into drill bit sizes, let's clarify what is M6 in the first place. The "M" in M6 screw stands for "metric," indicating it follows the ISO metric screw thread standard. The number "6" refers to the nominal major diameter of the screw in millimeters. So an M6 screw has a thread diameter of approximately 6mm.

But here's where it gets a bit more technical: what is M6 bolt size exactly? The actual major diameter (the outermost thread measurement) of an M6 screw is typically 6.0mm, but the exact measurement can vary slightly depending on tolerance class. The pitch—the distance between adjacent threads—is also important. For a standard M6 screw, the pitch is 1.0mm (coarse thread, designated M6×1.0) or 0.75mm (fine thread, M6×0.75).

This brings us to the M6 screw dimensions you need to know:

• Major diameter: 6.0mm (0.236 inches)
• Pitch: 1.0mm standard (or 0.75mm fine)
• Thread angle: 60° (standard for metric threads)

The Two Types of Holes: Tap vs. Clearance

When preparing for an M6 screw, you typically need one of two hole types:

1. Tapped Holes (for creating internal threads)

When you need to screw directly into a material (like metal, plastic, or wood), you create a tapped hole. This involves drilling a slightly smaller hole (the "tap drill size") and then using a tap to cut threads into the material. The M6 screw will then thread directly into these created threads.

2. Clearance Holes (for bolts passing through)

When your M6 screw needs to pass through one material to be secured with a nut on the other side, you create a clearance hole. This hole is slightly larger than the screw's maximum diameter, allowing it to pass through freely without engaging threads.

What Size Drill Bit for an M6 Tap Hole?

This is the heart of our guide. To create threads for an M6 screw, you need to drill a hole that's smaller than 6mm—but how much smaller?

The Golden Rule: 85-90% Rule

For metric coarse threads, a general rule is that the tap drill size should be approximately 85-90% of the screw's major diameter. For M6 screws, this means your tap drill should be about 5.1mm to 5.4mm in diameter.

The Exact Tap Drill Sizes for M6 Screws

Here are the precise drill bit sizes based on thread type:

This formula gives you approximately 75% thread engagement, which is considered optimal for most applications. For aluminum or softer materials, you might go slightly larger (like 5.1mm) to make tapping easier. For maximum strength in steel, stick with 5.0mm.

Thread Engagement Percentage Explained

• 60-65% thread engagement: Easier to tap, adequate for many applications
• 75% thread engagement: Industry standard for most applications (optimal strength vs. effort)
• 85%+ thread engagement: Maximum strength but difficult to tap, higher risk of tap breakage

What Size Drill Bit for an M6 Clearance Hole?

When your M6 screw needs to pass through material without threading into it, you need a clearance hole. Unlike tap holes, clearance holes are larger than the screw diameter.

Standard Clearance Hole Sizes for M6 Screws

For most DIY and general workshop applications, the 6.6mm drill bit is your go-to choice for M6 clearance holes.

Why 6.6mm?

• The M6 screw's major diameter is approximately 6.0mm
• Manufacturing tolerances mean screws can be slightly larger (up to ~6.1mm)
• A 6.6mm hole gives about 0.5mm (0.020") clearance all around—enough for easy insertion but not so much as to cause misalignment

Visual Reference: M6 Screw Drill Sizes Chart

To make this crystal clear, here's a quick-reference chart for M6 screw hole sizes:

Practical Guide: Step-by-Step Instructions

Creating a Tapped Hole for M6 Screws

1. Mark your hole location accurately with a center punch
2. Secure your workpiece in a vise or clamp
3. Select your drill bit: 5.0mm for M6×1.0 coarse threads
4. Drill the hole perpendicular to the surface, using cutting fluid for metals
5. Deburr the hole with a countersink or larger drill bit
6. Select the correct tap: M6×1.0 for coarse threads
7. Tap the hole: Start straight, use tapping fluid, turn forward 1-2 turns, then back ½ turn to break chips
8. Clean the threads with compressed air or a brush

Creating a Clearance Hole for M6 Screws

1. Mark hole locations on all pieces to be joined
2. Clamp pieces together for alignment
3. Select your drill bit: 6.6mm for standard clearance
4. Drill through all layers in one operation for perfect alignment
5. Deburr both sides of the hole
6. Test fit your M6 screw—it should pass through freely without forcing

Material-Specific Considerations

Steel and Hard Metals

• Use the standard 5.0mm tap drill for M6×1.0
• Always use cutting/tapping fluid
• Consider slightly larger (5.1mm) if tapping by hand in hardened steel

Aluminum and Soft Metals

• Use 5.1-5.2mm tap drill for easier tapping
• Be extra careful—aluminum is soft and can gall
• Consider thread-forming taps instead of thread-cutting for better results

Wood

• For hardwood: Use 5.0mm tap drill for M6 threads
• For softwood: You can often skip tapping—the M6 screw will create its own threads
• For clearance holes in wood: 6.6mm works, but wood can compress, so you might go as small as 6.2mm

Plastics

• Use 5.1-5.2mm tap drill to avoid cracking
• Drill and tap slowly to minimize heat buildup
• Consider thread-forming screws designed for plastic

Common Mistakes and How to Avoid Them

1. Using the wrong drill size for tapping

Problem: Too small = broken tap; too large = weak threads

Solution: Always use the recommended 5.0mm for M6×1.0

2. Drilling at an angle

Problem: Screws won't sit straight, threads won't engage properly

Solution: Use a drill press when possible, or a drilling guide for hand drills

3. Not using lubrication

Problem: Excessive heat, poor thread quality, broken taps

Solution: Always use appropriate cutting fluid for the material

4. Insufficient clearance

Problem: Screw won't pass through, forcing it damages threads

Solution: When in doubt, test with a spare M6 screw before assembly

Advanced Topics: When to Deviate from Standard Sizes

Thread Depth Considerations

The standard rule for thread depth is 1.5 to 2 times the diameter. For M6 screws, this means:

• Minimum thread engagement: 9mm
• Recommended thread engagement: 12mm
• Maximum practical engagement: 12-15mm (beyond this provides little additional strength)

For Through-Hardened Materials

When tapping into very hard materials, consider:

• Using a spiral flute tap for through-holes
• Increasing tap drill size by 0.1mm to reduce tapping force
• Using premium quality taps (TiN coated) for better wear resistance

For Thin Materials

When material thickness is less than the screw diameter:

• Consider threaded inserts instead of tapping directly
• For clearance holes, you might reduce size slightly to maintain material strength

Essential Tools for Working with M6 Screws

To work effectively with M6 screws, you'll need:

• Drill bits: 5.0mm (for tapping), 6.6mm (for clearance)
• Taps: M6×1.0 tap set (taper, plug, and bottoming taps if needed)
• Tap wrench: For holding and turning taps
• Cutting fluid: Appropriate for your material
• Depth stop: To ensure consistent hole depth
• Deburring tool: To clean hole edges
• Thread gauge: To verify thread pitch if unsure

Conclusion: Your M6 Screw Hole Cheat Sheet

To answer our original question "What size drill bit for an M6 screw?" once and for all:

• For tapped holes (creating threads): Use a 5.0mm drill bit for standard M6×1.0 screws
• For clearance holes (bolts passing through): Use a 6.6mm drill bit for standard applications

Understanding what is M6 and the precise M6 bolt size is just the beginning. By using the correct drill sizes for both tapping and clearance applications, you'll ensure strong, reliable connections in all your projects. Remember that while these are the standard recommendations, always consider your specific material and application requirements.

Whether you're building furniture, working on automotive projects, or tackling home improvements, getting your M6 screw holes right the first time saves frustration, materials, and most importantly—time. Now that you know exactly what size drill bit for an M6 screw, you're ready to tackle any project with confidence!

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