Osg 1297900 Spiral Flute Extension Tap 1/4-20 Unc H3 Hss

Product Description

The OSG 1297900 is a spiral flute extension tap designed for right-hand threading in blind-hole applications requiring a bottoming chamfer. Built from high-speed steel with a bright, uncoated finish, it cuts a 1/4-20 UNC thread to an H3 thread limit and a 3B class of fit. The 50-degree helix angle promotes efficient chip evacuation up the flute, making it well suited for materials that produce stringy chips. At 4 inches overall length with a reduced shank, this extension tap reaches into recessed or deep-set bosses that standard-length taps cannot access. Three flutes and a 0.255-inch shank diameter are sized to work with standard tap holders and extension attachments.

This tap is intended for production and maintenance threading operations in aluminum, carbon steel, and cast iron. The bottoming chamfer allows threads to be cut close to the bottom of a blind hole, which is a common requirement in machined components, fixtures, and fabricated assemblies. The extension length is particularly useful when tapping through workpiece fixtures or when the tap must reach past an obstruction. It suits job shops, toolrooms, and field maintenance environments where a range of materials and tight-tolerance thread fits are encountered regularly.

Key Features

• Spiral flute design with 50-degree helix angle for controlled chip removal in blind holes
• Bottoming chamfer allows threading to the base of blind holes
• H3 thread limit and 3B class of fit for standard tolerance threading
• Reduced shank at 0.255 inches diameter enables use with extension tap holders
• High-speed steel construction rated for aluminum, carbon steel, and cast iron
• 4 inches overall length with a 1-inch thread length for extended reach applications

Compatibility

Designed as an OEM-style replacement or direct-use spiral flute extension tap for 1/4-20 UNC threading applications in standard tap holders and extension drive systems accepting a 0.255-inch shank diameter.

Installation Notes

This tap is installed and driven by a licensed machinist or qualified maintenance technician using a tap wrench, tapping head, or CNC rigid-tapping setup appropriate for the workpiece material. Secure the workpiece fully before beginning the threading operation and ensure the tap is aligned perpendicular to the hole axis to avoid tap breakage. Use cutting fluid rated for the workpiece material, particularly when tapping carbon steel or cast iron. Do not exceed the recommended tap drill size for 1/4-20 UNC, and back off periodically in manual operations to break and clear chips.