Countersunk Allen Key Socket Screws M3–M10 — Flush-Fit Fastening with Grade 12.9 Strength
When a protruding screw head creates a snag point, clearance problem, or simply looks unfinished, you need a fastener that disappears into the surface. Machinists, fabricators, furniture builders, and serious DIYers who need a clean, flush-mount connection rely on countersunk Allen key socket screws to get the job done right — without sacrificing holding strength.
Each screw in this listing is a flat-head (90° countersunk) internal hex socket screw rated Grade 12.9 high tensile alloy steel. Available diameters span M3 through M10, with lengths (including head) from 6mm (≈1/4") to 50mm (≈2"). Pack quantities of 10, 20, 50, or 100 pieces let you order exactly what your project requires — no more, no less.
Typical applications include CNC machine bed assembly, automotive bracket mounting, precision fixture building, electronics enclosure assembly, and furniture hardware installation — anywhere you need a flat head hex socket screw that sits flush while holding under significant load.
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Compatibility: ISO metric coarse thread — M3 through M10. Fits any matching metric nut, threaded insert, or tapped hole per ISO 68-1. Hex key sizes: M3→2mm, M4→2.5mm, M5→3mm, M6→4mm, M8→5mm, M10→6mm.
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Note: Grade 12.9 alloy steel is not stainless. Not recommended for outdoor or corrosive environments without a protective coating. Contact us if you need guidance on suitable alternatives.
✅ Key Features of These Countersunk Allen Key Socket Screws M3–M10
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💪Grade 12.9 High Tensile Alloy Steel — Minimum tensile strength of 1220 MPa makes these flat head hex socket screws suitable for high-load mechanical assemblies where standard Grade 8.8 fasteners would fail under stress.
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🔩90° Flat Countersunk Head — The flush-sitting head eliminates snag points and allows smooth movement of adjacent components — ideal for machine slides, panel assemblies, and precision fixtures.
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🔑Internal Hex (Allen Key) Drive — The recessed socket allows torque application in confined spaces where a standard wrench cannot reach, making installation faster and more controlled in tight assemblies.
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📏Wide Size Range — M3 to M10, 6mm to 50mm — A single listing covers the most common metric countersunk screw sizes used in automotive, industrial, and DIY applications, reducing the need to source from multiple suppliers.
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📦Flexible Pack Quantities — 10 to 100 Pieces — Choose from 10-, 20-, 50-, or 100-piece packs to match your exact project quantity, whether you're doing a one-off repair or stocking a workshop supply.
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⚙️ISO Metric Coarse Thread Standard — Compatible with off-the-shelf metric nuts, threaded inserts, and tapped holes across automotive, fabrication, and electronics applications — no special tooling required.
📐 Technical Specifications — Flat Head Countersunk Socket Screws
| Head Type |
Flat countersunk (90° included angle) |
| Drive Type |
Internal hex socket (Allen key) |
| Thread Standard |
ISO metric coarse thread |
| Available Diameters |
M3, M4, M5, M6, M8, M10 |
| Available Lengths (incl. head) |
6mm (≈1/4") — 50mm (≈2") |
| Hex Key Sizes |
M3→2mm | M4→2.5mm | M5→3mm | M6→4mm | M8→5mm | M10→6mm |
| Material |
Alloy steel |
| Grade / Strength Class |
12.9 (Tensile strength ≥ 1220 MPa) |
| Surface Finish |
See variant options |
| Pack Quantity |
10 / 20 / 50 / 100 pcs (select above) |
| Thread Pitch |
Contact us to confirm per diameter (ISO metric coarse standard) |
❓ Frequently Asked Questions
How do I choose the right diameter and length for countersunk Allen key screws?
Match the diameter to your tapped hole or nut size (e.g., M5 for an M5 threaded hole). For length, measure the total thickness of the material being fastened and add the countersink depth so the head sits flush. For example, M5 x 16mm (≈5/8") is typically used for joining two 6–7mm panels. Lengths in this listing run from 6mm (≈1/4") to 50mm (≈2") — select the size that gives you at least 1.5× the thread diameter of engagement in the tapped material.
What does Grade 12.9 mean and how strong are these screws?
Grade 12.9 is among the highest strength classifications for metric fasteners, indicating a minimum tensile strength of 1220 MPa and a proof load of approximately 1100 MPa. These screws are generally used in high-stress mechanical assemblies, heavy machinery, and structural applications where standard Grade 8.8 fasteners are insufficient.
Are these countersunk screws suitable for outdoor or corrosive environments?
Grade 12.9 alloy steel is not inherently corrosion-resistant. For outdoor or humid environments, applying a corrosion-inhibiting coating or using stainless steel alternatives is generally advisable. If outdoor use is required, pairing with anti-seize compound or a zinc-based primer can help extend service life. Contact us if you need guidance on suitable alternatives.
What thread standard do these screws use — are they compatible with standard metric nuts and inserts?
These screws use standard ISO metric coarse thread (e.g., M5 × 0.8mm pitch, M8 × 1.25mm pitch). They are compatible with any matching metric nut, threaded insert, or tapped hole following ISO 68-1 thread standards — widely used in automotive, industrial machinery, and fabrication applications worldwide.
What Allen key (hex key) size do I need for each screw diameter?
Hex key sizes typically correspond to the screw diameter: M3 generally uses a 2mm key, M4 uses 2.5mm, M5 uses 3mm, M6 uses 4mm, M8 uses 5mm, and M10 uses 6mm. Always verify with a test fit before applying full torque to avoid rounding the socket drive.
What is the countersink angle on these flat head screws?
Flat head countersunk socket screws in this style typically feature a 90° included angle countersink, which is the standard for ISO metric countersunk screws. This allows the head to sit flush with or below the mating surface when used with a matching countersunk hole — no modification required for standard countersink drill bits.
How do I choose between a 10-pack and a 100-pack?
For one-off repairs, prototyping, or testing fit, the 10- or 20-piece packs let you order exactly what you need without over-buying. For production runs, maintenance kits, or workshop stock, the 50- or 100-piece packs offer a lower per-unit cost. Select your quantity from the Quantity(Piece) option above.
🛒 Select your screw diameter, length, and quantity above, then add to cart — ships from our US warehouse.
