The LD-391 alloy steel socket head cap screw series, commonly known as Allen bolts, delivers high tensile strength and a compact low-profile head for precision fastening across machinery, automotive, and tooling applications. Available in diameters from M3 to M16 and lengths of 8 to 100mm, these screws are manufactured to the DIN 912 / ISO 4762 dimensional standard, ensuring consistent head height, thread pitch, and hex socket size for interchangeability with global hardware. The cylindrical head sits flush within a counterbored hole, providing a smooth bearing surface that distributes clamping load evenly and keeps the drive recess protected from contamination.

Material selection is central to the performance of this series. Each screw is produced from alloy steel heat-treated to Class 10.9 or Class 12.9, offering tensile strengths of approximately 1040 MPa and 1200 MPa respectively. Class 12.9 is specified for high-stress joints such as engine mounts and mold clamping, while Class 10.9 balances strength and cost for general machinery assembly. The standard black oxide finish provides a thin, uniform conversion coating that imparts mild corrosion resistance, reduces galling during repeated assembly, and gives the fastener a consistent dark appearance. Where greater corrosion protection is required, white or yellow zinc plating and stainless steel 304/316 options are available.

The hex socket (Allen) drive is a defining advantage of the socket head cap screw. Compared with external hex bolts, the internal hex drive allows higher torque transfer in a smaller head diameter, making these screws ideal for confined spaces and closely spaced hole patterns. The recessed drive also permits the screw to be seated within a counterbore so the head sits below the surface, leaving adjacent components unobstructed. Full-thread and partial-thread options are offered: full-thread screws engage the full clamping length for tensile joints, while partial-thread versions provide a smooth shank for shear-load applications and precise clamping of stacked components.

For wholesale buyers and OEM accounts, the LD-391 series supports extensive customization to match project requirements. OEM sizes and grades, custom plating, knurled head options for finger-adjustable applications, and private-label packaging can all be supplied. With a minimum order quantity of 1000 pieces, mixed orders across sizes and finishes supported, and a production lead time of 7 to 15 days, the LD-391 series offers importers and manufacturers a reliable, standards-compliant source of alloy steel socket head cap screws at competitive wholesale pricing.