FF-B-561 Lag Bolts (Screw)

The original specification for FF-B-561 lag bolts outlined heavy-duty externally threaded fasteners engineered with a gimlet or cone point. These bolts are driven by torquing the head rather than fastening a nut. They are used primarily for anchoring metal to wood, or heavy timber connections, where a pre-drilled pilot hole receives the coarse lag thread for maximum grip.

Like many hardware-focused federal specifications, the military phased out this standard in favor of equivalent commercial consensus standards. The final government revision directs contractors and buyers to ASME standard documentation, specifically ASME/ANSI B18.2.1 (Square and Hex Bolts and Screws) and ASME/ANSI B18.18.1M (Quality Assurance for General Purpose Fasteners).

• FF-B-561 lag bolts, part numbers, and NSNs still frequently appear on legacy engineering drawings, defense hardware schedules, and older government stock records.
• Modern procurement contracts should translate these outdated callouts to their modern ASME structural equivalents to ensure current manufacturing compliance.

When sourcing replacements for legacy FF-B-561 lag bolts, material grades and protective finishes must match the demands of the operating environment. While the original federal spec covered basic carbon steel and limited alloys, modern manufacturing allows for significant upgrades in corrosion resistance and tensile strength.

Common modern adaptations include hot-dip galvanized steel for exterior timber construction, marine-grade stainless steel (316L) for highly corrosive maritime applications, and specialized PTFE coatings for industrial infrastructure. Regardless of the material chosen, the threads must maintain the aggressive pitch designed to bite deep into structural wood without shearing.