MIL-F-18240 Self-Locking Fasteners — Element (Legacy)
The legacy specification for MIL-F-18240 self-locking fasteners originally defined the self-locking thread element used on externally threaded bolts and screws for service up to 250°F. Its final issue, MIL-F-18240 revision E (1 December 1989), was superseded on June 2, 1997, by MIL-DTL-18240, which remains the active governing document for these Federal Specification fasteners today.
At a Glance
MIL-F-18240 Self-Locking Fasteners Overview
MIL-F-18240 was the original specification covering the performance criteria of any self-locking thread element incorporated into externally threaded fasteners. These specialized bolts and screws were engineered to maintain their torque and resist extreme vibration in applications where operational temperatures do not exceed 250°F, serving as a reliable alternative to traditional lock-wiring.
The specification went through several revisions between the early 1970s and 1989, when MIL-F-18240 revision E became the final "MIL-F" issue. On June 2, 1997, the Department of Defense formally converted the specification to the "DTL" (detail specification) format as MIL-DTL-18240, Revision F. Despite the designation change, the core technical scope—covering self-locking elements for UNC, UNJC, UNF, UNJF, UNRC, or UNRF threaded fasteners in strip or patch configurations—carried forward essentially unchanged.
Applications and Transition to MIL-DTL-18240
For decades, these self-locking fasteners were the standard choice for replacing cumbersome lock-wiring in aerospace and heavy industrial applications. The self-locking thread element—often applied as a nylon strip or patch directly onto the threads—provided reliable prevailing torque that prevented loosening under severe vibration without exceeding the 250°F limit.
When the Department of Defense shifted from standard military specifications to detail specifications, the standard officially transitioned to MIL-DTL-18240. However, because the underlying technology remained identical, historical documentation, legacy drawings, and even modern patent literature frequently refer to the older MIL-F-18240 designation. For the most current qualification testing and amendment history on these Federal Specification fasteners, engineers should consult the Defense Logistics Agency (DLA) ASSIST portal.
Specification Lineage
| Document | Status |
|---|---|
| MIL-F-18240 (through Rev. E, 1989) | Superseded by MIL-DTL-18240 |
| MIL-DTL-18240F (1997–present) | Active, current governing document |
MIL-F-18240 is superseded. Current and future procurement should reference MIL-DTL-18240, which covers the same self-locking thread element under its current "DTL" designation. This page is retained for legacy drawing cross-reference only.
Frequently Asked Questions
Is MIL-F-18240 the same as MIL-DTL-18240?
They cover the same self-locking thread element technology. MIL-DTL-18240 is the current, active designation that superseded MIL-F-18240E in 1997 when the document was converted to the "DTL" specification format.
Why do patents and older documents still cite MIL-F-18240?
Documents written before 1997 — including some still-referenced patents on locking-element technology — predate the conversion to MIL-DTL-18240 and retain the original designation. Parts should now be procured under the active spec.
Need Self-Locking Fasteners to MIL-DTL-18240?
Ananka Fasteners supplies bolts and screws with current MIL-DTL-18240 self-locking elements, fully tested and certified.
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