Left-Hand Reverse Thread Hex Socket Cap Screws M4–M16 Black

Reverse Thread Hex Socket Cap Screws — Black 12.9 Grade Steel

These left-handed (reverse thread) hex socket head cap screws are engineered for applications where standard right-hand threads would loosen under rotational forces — such as motor shafts, pedal axles, grinder spindles, and rotating machinery components. Available in sizes M4 through M16, they are manufactured from 12.9 grade high-strength carbon steel with a black oxide surface finish for a sleek appearance and added corrosion resistance.

Key Specifications

• Thread Direction: Left-handed (reverse/counter-clockwise thread)
• Material: 12.9 Grade High-Strength Carbon Steel
• Surface Finish: Blackened (Black Oxide)
• Available Sizes: M4, M5, M6, M8, M10, M12, M14, M16 (thread diameter 4 mm–16 mm)
• Thread Length: 10 mm – 80 mm options available
• Drive Type: Hex (Allen) Socket Head
• Quantity per Lot: 1–10 pcs (selected at checkout)
• Packing: Plastic bag / carton
• Country of Origin: Made in China

Why Choose Left-Hand Thread Screws?

Left-hand thread fasteners are the go-to solution in mechanical assemblies where vibration or rotational motion would otherwise cause a standard right-hand bolt to back out over time. By tightening in the counter-clockwise direction, they remain securely locked in place — no thread-locking compounds required. Grade 12.9 construction provides exceptional tensile strength, making these ideal for heavy-duty metalworking, automotive, machinery, and DIY fabrication projects.

Common Applications

• Angle grinder spindles and cutting tool arbors
• Bicycle pedal axles (left-side pedals)
• Motor shafts and rotating equipment
• CNC machine fixtures and tooling
• Custom fabrication and metalworking projects

Size Guide

Sizes are listed as d × L — where d is the thread diameter and L is the thread length. For example, M4 × 10 mm means a 4 mm thread diameter bolt with a 10 mm thread length. Please refer to the product images in the gallery for a full dimensional chart. If you require a size or quantity not listed, feel free to contact us for a custom order.

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Frequently Asked Questions

Q: What is a left-hand thread (reverse thread) bolt and when do I need one?

A: A left-hand thread bolt tightens counter-clockwise and loosens clockwise — the opposite of a standard bolt. They are used in applications where rotation during operation would cause a right-hand thread to loosen, such as on the left-side pedal of a bicycle, grinder spindles, or any rotating shaft assembly where self-loosening is a concern.

Q: What does 12.9 grade mean for these hex socket cap screws?

A: Grade 12.9 is among the highest strength classifications for metric steel fasteners, indicating a minimum tensile strength of 1220 MPa. This makes these screws suitable for high-stress, heavy-duty metalworking and industrial applications where standard-grade hardware would not be adequate.

Q: How do I identify a left-hand thread screw to avoid mixing it up with a standard one?

A: Left-hand thread screws are often marked with an "L" or "LH" designation on the head or packaging. Visually, the threads spiral in the opposite direction from a standard right-hand screw. These black oxide-finished bolts should be stored separately and clearly labeled to prevent accidental mix-ups during assembly.

Q: Can I order custom lengths or bulk quantities of these reverse thread hex socket screws?

A: Yes — if you need a size combination, length, or quantity not listed in the standard options, please contact us directly. Bulk purchase pricing and custom specifications are available upon request.

Production & Packaging Notice

To maintain high stock availability and fast shipping times, our items are sourced from multiple verified manufacturing facilities. While the core quality, materials, and product specifications remain identical, you may notice slight variations in the packaging design, manufacturing origin, or the wording of the printed instructions.