MIL-F-18240 — Self-Locking Fastener Element (Legacy)

MIL-F-18240 originally defined the self-locking thread element used on externally threaded fasteners for service up to 250°F. Its final issue, Revision E (1 December 1989), was superseded on June 2, 1997 by MIL-DTL-18240F, which is the active governing document today.

Status: Superseded — see MIL-DTL-18240 Type: Self-Locking Thread Element Last Revision: E, 1 Dec 1989 Max Temp: 250°F

At a Glance

Full Title

Fastener Element, Self-Locking, Threaded Fastener, 250°F Max

Superseded By

MIL-DTL-18240F (2 Jun 1997)

Thread Types

UNC, UNJC, UNF, UNJF, UNRC, UNRF

Configurations

Strip & Patch (Types L / P)

FSC Class

53GP

First Issued

1972 (earliest known revision)

Historical Use

Replacement for lock-wired bolts/screws

Current Guidance

Specify MIL-DTL-18240 for new designs

Overview

MIL-F-18240 was the original specification covering self-locking elements incorporated into the threads of externally threaded fasteners — bolts and screws — used in applications where temperatures do not exceed 250°F, as a lock-wiring alternative. The specification went through several revisions between the early 1970s and 1989, when Revision E became the final "MIL-F" issue.

On June 2, 1997, the Department of Defense converted the specification to the "DTL" (detail specification) format as MIL-DTL-18240, Revision F, which superseded MIL-F-18240E and remains the active governing document. The technical scope — self-locking elements for UNC, UNJC, UNF, UNJF, UNRC, or UNRF threaded fasteners, in strip or patch configurations — carried forward essentially unchanged through the conversion.

Some patent literature and older engineering documentation still references "MIL-F-18240" by name when describing the underlying locking-element technology, even though the governing document has since moved to MIL-DTL-18240.

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 E, 1989)	Superseded by MIL-DTL-18240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MIL-F-18240 is superseded.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.

MIL-F-18240 — 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 MIL-DTL-18240 is the current, active designation that superseded MIL-F-18240E in 1997 when the document was converted to the "DTL" specification format.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.

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