Heli-coil A3591-4CN250 Thread Insert 1/4-28 Unf 1D

Product Description

The HELI-COIL A3591-4CN250 is a screw-locking, tangless wire thread insert designed to restore or reinforce internal threads in parent materials that see sustained vibration or repeated fastener cycling. Manufactured from Grade 304 stainless steel with a bright finish, this insert installs into a prepared tapped hole and provides a hardened, corrosion-resistant thread form rated for up to 30 inch-pounds maximum locking torque. The tangless design eliminates tang removal after installation, reducing steps and the risk of debris in blind holes. At a 1D length-to-diameter ratio and 0.250 inches overall length, it suits applications where thread depth is limited.

This insert is used in metalworking, maintenance, and repair applications across industrial, aerospace, and mechanical service environments. Typical use cases include thread repair in aluminum, magnesium, or cast housings, and original equipment manufacturing where a hardened thread form is required in softer base materials. The 1/4-28 UNF thread size fits fine-pitch fastener applications common in precision assemblies. Installation requires a properly sized HELI-COIL tap and compatible tangless installation tool. Sold individually.

Key Features

• Tangless design eliminates post-installation tang removal in blind or through holes
• Grade 304 stainless steel construction resists corrosion in wet or chemically exposed environments
• Maximum locking torque of 30 inch-pounds provides reliable thread locking under vibration
• Minimum outside diameter of 0.306 inches and maximum of 0.326 inches for controlled fit in the tapped parent bore
• 5 free coils with a 1D length-to-diameter ratio suited to shallow thread engagement applications
• Bright finish with no coating additions that could affect dimensional tolerances

Compatibility

Designed as an OEM-style and repair thread insert for 1/4-28 UNF tapped holes prepared with a HELI-COIL specification tap to match the 1D insert length series.

Installation Notes

Installation is performed by machinists or maintenance technicians using a HELI-COIL specification tap to prepare the parent bore, followed by a tangless insertion tool matched to the 1/4-28 UNF insert series. Ensure the parent material is securely fixtured and the tapped hole is clean and free of chips or coolant before inserting. Do not use standard UNF taps in place of the required HELI-COIL oversized tap, as the insert will not seat correctly. Follow the manufacturer's installation tooling specifications for proper coil engagement depth.