Osg 1297800 Spiral Flute Extension Tap 1/4-20 Unc H3

Product Description

The OSG 1297800 is a spiral flute extension tap designed for right-hand threading in a range of common materials including aluminum, carbon steel, and cast iron. Built from high-speed steel with a bright, uncoated finish, this tap features a 50-degree helix angle that promotes efficient chip evacuation during blind-hole tapping operations. The plug chamfer style provides a gradual lead-in, making it well suited for through-hole and moderate-depth blind-hole applications. At 4 inches overall length, the extension design provides added reach in confined or recessed locations where a standard-length tap cannot seat properly. Licensed machinists and maintenance technicians are the primary users of this tooling.

This tap cuts a 1/4-20 UNC thread to a 3B class of fit with an H3 thread limit, offering a tolerance range appropriate for general-purpose threaded assemblies. The reduced shank configuration and 0.255-inch shank diameter allow it to be used in standard tap holders and chucks. It is suited for production and maintenance threading in job shops, manufacturing facilities, and field maintenance environments where extended reach is required for hard-to-access workpieces.

Key Features

• Spiral flute design with 50-degree helix angle for positive chip evacuation in blind and through holes
• High-speed steel construction provides durability across aluminum, carbon steel, and cast iron applications
• Plug chamfer with 3-flute geometry for smooth thread engagement and controlled cutting action
• Extension length of 4 inches overall enables reach into recessed or deep-bore locations
• H3 thread limit and 3B class of fit meet standard tolerance requirements for general threaded fastener assemblies
• Reduced shank with 0.191-inch square size for compatibility with standard tap wrenches and holders

Compatibility

Designed for use in standard tap holders, T-handle tap wrenches, and machine tap chucks accepting a 0.255-inch shank diameter. Suitable for threading aluminum, carbon steel, and cast iron workpieces to a 1/4-20 UNC thread form.

Installation Notes

This tap should be installed and operated by qualified machinists or maintenance technicians familiar with threading operations and cutting tool safety. Ensure the workpiece is securely clamped and properly piloted before beginning the tapping cycle. Use an appropriate cutting fluid for the material being threaded to extend tool life and maintain thread quality. Inspect the tap for wear or damage before each use and never force a tap that has stopped advancing under normal cutting load.