Understanding m20 countersunk bolt dimensions is essential when you need a flush, professional fastening solution for structural, industrial, or architectural applications. Whether you’re fixing steel plates, securing brackets, or finishing a project where safety and aesthetics matter, choosing the correct M20 countersunk bolt ensures strength, compliance, and long-term performance.

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What Is an M20 Countersunk Bolt?

An M20 countersunk bolt is a metric fastener with:

• A 20mm nominal thread diameter

• A countersunk (flat) head designed to sit flush with or below the surface

• Typically manufactured to DIN 7991 or ISO 10642 standards

Countersunk bolts are commonly used where:

• A smooth surface finish is required

• Protruding bolt heads would cause interference

• Safety, appearance, or aerodynamics are important

At Blue Croc Shop, M20 countersunk bolts are commonly supplied in stainless steel and high-tensile steel, suitable for trade and industrial use.

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Standard M20 Countersunk Bolt Dimensions

Below are the standard dimensions you should understand before ordering or specifying an M20 countersunk bolt.

Nominal Thread Diameter

• 20mm (M20)

This is the external diameter of the bolt thread.

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Head Diameter (dk)

• Maximum: ~36.0 mm

• Minimum: ~35.0 mm

The head diameter determines how wide the countersunk recess must be machined into the material.

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Head Angle

• 90° countersunk angle

This is critical. The mating hole must also be cut to a 90-degree countersink to ensure full surface contact and load distribution.

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Head Height (k)

• Approximately 11.0 mm

This is the vertical depth of the countersunk head from the surface to where the shank begins.

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Thread Pitch

• Standard coarse pitch: 2.5 mm

This means the thread advances 2.5 mm per full rotation. Coarse pitch threads are stronger and more resistant to stripping in steel and structural materials.

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Thread Length

• Fully threaded for shorter bolts

• Partially threaded for longer lengths

Thread length depends on the overall bolt length and the manufacturing standard.

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Common M20 Countersunk Bolt Lengths

Blue Croc Shop supplies M20 countersunk bolts in a range of lengths to suit different material thicknesses.

Typical lengths include:

• 30mm

• 40mm

• 50mm

• 60mm

• 70mm

• 80mm

• 90mm

• 100mm and longer

Tip: Always allow for:

• Material thickness

• Countersunk depth

• Washer or nut engagement (minimum 1–1.5 thread diameters past the nut)

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M20 Countersunk Bolt Hex Drive Size

Most M20 countersunk bolts use an internal hex (Allen) drive.

• Hex socket size: 12 mm (commonly)

This ensures high torque transfer while maintaining a flush surface finish.

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Material Options at Blue Croc Shop

Choosing the correct material is just as important as selecting the right dimensions.

Stainless Steel M20 Countersunk Bolts

Best for:

• Outdoor environments

• Marine or coastal applications

• Corrosion resistance

Common grades:

• 304 Stainless Steel (A2) – General-purpose corrosion resistance

• 316 Stainless Steel (A4) – Superior corrosion resistance for harsh environments

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High Tensile Steel M20 Countersunk Bolts

Best for:

• Structural steel

• Heavy load-bearing applications

• Engineering and construction projects

Typical grades:

• 8.8 – Strong, widely used in construction

• 10.9 / 12.9 – High-strength applications requiring maximum load capacity

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Why Countersunk Bolt Dimensions Matter

Incorrect countersunk bolt dimensions can cause:

• Poor load distribution

• Head pull-through

• Cracking around the countersink

• Bolt loosening over time

Correct m20 countersunk bolt dimensions ensure:

• Even clamping force

• Full head-to-material contact

• Structural integrity

• Professional finish

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How to Measure an M20 Countersunk Bolt Correctly

Measuring Bolt Length

For countersunk bolts, length is measured from the top of the head to the end of the bolt, not from under the head.

This is a common mistake and can lead to ordering bolts that are too short.

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Measuring Countersink Depth

• Depth should match the head height (~11 mm)

• Angle must be 90 degrees

• Hole diameter must match the head diameter

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Typical Applications for M20 Countersunk Bolts

M20 countersunk bolts supplied by Blue Croc Shop are commonly used in:

• Structural steel assemblies

• Machinery mounting plates

• Architectural steelwork

• Bridge components

• Heavy-duty brackets

• Fabricated frames

• Flush floor plates

These applications demand precise dimensions, strength, and reliability.

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M20 Countersunk Bolt vs Hex Bolt

If a flush finish is required, countersunk bolts are the correct choice.

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Installation Tips for M20 Countersunk Bolts

To ensure correct installation:

• Use a 90° countersink cutter

• Match the countersink diameter precisely

• Use a calibrated torque wrench

• Avoid over-tightening, especially in stainless steel

• Apply anti-seize to stainless bolts to prevent galling

Correct installation extends bolt life and prevents joint failure.

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Why Buy M20 Countersunk Bolts from Blue Croc Shop?

When purchasing from www.bluecrocshop.com.au, you benefit from:

• Trade-grade fasteners

• Accurate dimensional specifications

• Australian-based supply

• Stainless and high-tensile options

• Consistent quality control

This ensures the M20 countersunk bolt dimensions you rely on are accurate and compliant with industry standards.

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Final Notes on M20 Countersunk Bolt Dimensions

Understanding m20 countersunk bolt dimensions allows you to:

• Specify correctly

• Reduce installation errors

• Improve safety and finish quality

• Avoid costly rework

For professional results, always match bolt dimensions, material grade, and installation method to the application.