M4–M8 Square Head Bolts Full Thread — Reliable Metric Fasteners for Woodworking, Machinery & Structural Assembly
When a standard hex bolt won't seat properly in a countersunk channel, or you need a bolt head that resists spinning in a square recess, the square head design solves the problem. These metric square head bolts are built for woodworkers, fabricators, machinists, and DIYers who need a dependable full-thread fastener that locks in place without slipping.
Each bolt is machined from 304 stainless steel with a full-thread shank — thread runs from tip to head — available in thread diameters M4, M5, M6, and M8, with lengths ranging from 12mm (≈1/2") to 80mm (≈3-1/8"). Packs are offered in quantities of 1, 5, 10, 15, and 20 pieces so you can stock exactly what your project requires.
Typical applications include timber frame joinery where the square head seats flush in a mortise slot, light machinery assembly requiring anti-rotation fasteners, greenhouse and garden structure hardware, and general-purpose metric bolting where a wrench-driven square head provides extra torque control.
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Thread Compatibility: ISO metric standard — fits all M4, M5, M6, and M8 metric nuts, washers, and tapped holes. Verify thread pitch with your mating hardware before ordering.
✅ Key Features of These M4–M8 Square Head Full Thread Bolts
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Square Head Design — The four-sided head seats securely in square-cut channels and timber slots, preventing rotation during tightening without needing a secondary anti-spin fixture.
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Full-Thread Shank — Thread runs the entire bolt length, maximizing engagement in through-holes and thin materials where a partial-thread bolt would leave an unthreaded gap.
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304 Stainless Steel Construction — Resists rust and surface oxidation in outdoor, humid, and mildly corrosive environments — a practical choice for garden structures, frames, and exposed assemblies.
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Metric Sizing M4–M8 — Four thread diameters cover a wide range of structural and mechanical fastening tasks, from light-duty M4 connections to heavier M8 load-bearing joints.
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Flexible Pack Quantities — Available in 1, 5, 10, 15, or 20-piece packs so you can match your order to the exact job size without overstocking.
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Grade 6.8 Strength Rating — Rated at approximately 600 MPa tensile strength, these bolts handle general structural and mechanical loads reliably across woodworking, fabrication, and assembly applications.
📐 Technical Specifications — Square Head Full Thread Metric Bolts
| Material |
304 Stainless Steel |
| Head Style |
Square |
| Thread Type |
Full Thread (thread runs entire shank length) |
| Thread Standard |
ISO Metric |
| Available Thread Diameters |
M4, M5, M6, M8 |
| Available Lengths |
12mm (≈1/2") – 80mm (≈3-1/8"); see variant options |
| Strength Grade |
6.8 |
| Finish |
Natural stainless (uncoated) |
| Pack Quantities |
1 / 5 / 10 / 15 / 20 pieces |
| Country of Origin |
China |
| Drive Method |
Wrench (open-end, adjustable, or socket) |
| Compatible Nuts / Washers |
See variant options |
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Confirm your thread pitch before ordering. Standard metric pitches apply (e.g., M6×1.0, M8×1.25), but verify with your mating nut or tapped hole if your application is pitch-sensitive.
❓ Frequently Asked Questions
How do I choose the right bolt length for my project?
Generally, your bolt length should equal the total thickness of the materials being joined plus enough thread engagement — typically 1.5× the bolt diameter. For example, joining two 10mm panels with an M6 bolt would typically call for a length of at least 30mm (≈1-3/16"). When in doubt, size up rather than down to ensure full nut engagement.
What thread standard do these square head bolts use?
These bolts use the ISO metric thread standard (M4, M5, M6, M8), compatible with standard metric nuts and tapped holes of the corresponding size. They are not interchangeable with UNC/UNF inch-standard hardware.
Can these bolts be used outdoors or in wet environments?
Yes. 304 stainless steel offers good corrosion resistance in most outdoor and mildly humid environments, including garden structures, greenhouses, and covered exterior assemblies. For highly corrosive or marine environments, 316 stainless steel is generally recommended instead.
What is the strength grade, and what loads can these bolts handle?
These bolts are Grade 6.8, indicating a tensile strength of approximately 600 MPa and a yield strength of around 480 MPa. They are suitable for general mechanical and structural fastening. For safety-critical or high-load structural applications, consult an engineer to confirm the appropriate fastener specification.
Do I need a special tool to tighten a square head bolt?
Square head bolts are typically driven with an open-end wrench, adjustable wrench, or a socket wrench sized to the head flats. They are not designed for screwdriver or hex key use. The square head also self-seats in square-cut timber slots, which can eliminate the need for a second person to hold the bolt during tightening.
Are these bolts fully threaded along the entire shank?
Yes — these are full-thread bolts. The thread runs the entire length of the shank from the underside of the head to the tip. This is particularly useful when fastening thin materials or when you need thread engagement across the full bolt length in a through-hole application.
What pack sizes are available, and can I mix sizes in one order?
Each listing variant is available in packs of 1, 5, 10, 15, or 20 pieces. To order multiple sizes or lengths, simply add each variant separately to your cart. Contact us if you need bulk quantities beyond what is listed.
🛒 Select your size and length above, then add to cart — ships from our US warehouse.
