Heli-coil A3591-4EN375 Screw-locking Insert 1/4-28 Unf

Product Description

The HELI-COIL A3591-4EN375 is a screw-locking wire thread insert designed to create a strong, vibration-resistant internal thread in parent materials where thread repair or reinforcement is required. Manufactured from stainless steel, this tangless insert measures 0.375 inches in overall length with a 1/4-28 UNF thread form and a length-to-diameter ratio of 1-1/2D. The locking feature relies on a deformed coil that applies radial pressure against the mating screw thread, resisting loosening under dynamic loads. Licensed machinists, maintenance technicians, and assembly professionals install this insert using standard Heli-Coil installation tooling.

This insert is suited for applications in aerospace, industrial equipment, and mechanical assemblies where a 1/4-28 UNF thread must hold under vibration or repeated fastener cycling. The tangless design eliminates the need for tang break-off after installation, simplifying the process in blind holes and reducing debris risk in precision assemblies. Typical use cases include thread repair in aluminum, cast iron, and other base materials where the parent thread has stripped or where added thread strength is specified by design.

Key Features

• Screw-locking design resists loosening under vibration with a maximum locking torque of 30 inch-pounds
• Tangless construction - no tang removal required after installation, reducing contamination risk in blind holes
• 1/4-28 UNF thread form, 1-1/2D length-to-diameter ratio
• Stainless steel construction for corrosion resistance in demanding environments
• Outside diameter range of 0.306 to 0.326 inches for precise fit in standard Heli-Coil tapped holes
• 8-1/4 free coils provide consistent locking engagement along the full insert length

Compatibility

Designed for use in standard Heli-Coil tapped holes sized for 1/4-28 UNF screw-locking tangless inserts with a 1-1/2D length specification. Requires the appropriate Heli-Coil tangless installation tool for proper seating.

Installation Notes

Installation should be performed by a qualified technician using the correct Heli-Coil tangless insertion tool matched to the 1/4-28 UNF thread size. The parent material must be drilled and tapped with a Heli-Coil tap to achieve the correct minor diameter before insert installation. No tang removal step is required with tangless inserts, but the installer should verify full seating depth and confirm the insert sits at or below the top of the parent thread. Follow applicable torque specifications from the fastener or assembly drawing - do not use generic torque values not specified for the assembly.