Osg 0146308 Spiral Flute Sti Tap 1/4-28 Unf Hss

Product Description

The OSG 0146308 is a spiral flute STI (Screw Thread Insert) tap designed for cutting 1/4-28 UNF internal threads in preparation for Heli-Coil or equivalent threaded insert installation. Built from high-speed steel with a V-coating finish, this tap features a 45-degree right-hand helix that promotes efficient chip evacuation in a variety of materials. The bottoming chamfer geometry allows threading close to the bottom of blind holes. With an overall length of 2-23/32 inches and a shank diameter of 0.318 inches, it fits standard tap holders used in machine shop and field repair environments. Licensed machinists, maintenance technicians, and tool-and-die professionals are the primary users.

This tap is applied in situations where a parent thread has been stripped or damaged and a threaded insert repair is required. It produces an oversize thread profile sized to accept standard STI inserts, restoring full thread strength in aluminum, cast iron, steel, and similar machinable materials. Typical applications include engine component repair, fixture rebuilding, and precision machined part rework in both production and maintenance settings. The 1-1/8 inch thread length accommodates common fastener engagement depths in those repairs.

Key Features

• 1/4-28 UNF thread form sized for STI insert installation
• 45-degree right-hand helix promotes upward chip flow for cleaner blind-hole tapping
• Bottoming chamfer allows threading to within 1-2 threads of hole bottom
• High-speed steel construction provides toughness across a broad range of workpiece materials
• V-coating finish reduces friction and extends tool life in general-purpose tapping
• H3 thread limit and 2B class of fit deliver consistent, accurate thread geometry for insert seating

Compatibility

Designed as an OEM-style STI tap for use with standard 1/4-28 UNF threaded insert systems; confirm insert manufacturer specifications to verify tap series compatibility before use.

Installation Notes

Always secure the workpiece fully before tapping and ensure the hole is drilled to the correct STI tap drill size for the insert being used. Use a quality tapping fluid appropriate for the workpiece material to extend tap life and improve thread finish. Apply consistent, steady feed pressure and back off periodically to break chips when tapping blind holes. Inspect the tap for wear or chipping before each use, as a damaged tap can compromise thread quality and make insert installation difficult.