MIL-I-8846 — Helical Coil Screw-Thread Inserts (Legacy)
MIL-I-8846 covered helical coil screw-thread inserts — commonly known by the trade name Heli-Coil — made from formed wire, whose inner surfaces provide threads once installed. Its final Revision D (8 March 1994, with Supplement 1 and Notice 1) was superseded on June 30, 1998 by NASM8846, the active governing document today.
At a Glance
Overview
MIL-I-8846 covered helical coil screw-thread inserts made from formed wire, whose inner surfaces, once assembled into a tapped hole, provide the finished internal thread. These inserts are intended primarily for original design applications — especially in soft materials such as aluminum and magnesium — to protect against thread failure caused by stripping, and are also widely used to repair damaged or worn-out threads.
The specification defined four types (coarse thread, fine thread, taper pipe thread, and metric spark plug thread) and multiple classes within each, including free-running inserts and self-locking "screw-locking" variants. The final MIL-I-8846D issue, including its Supplement 1, was superseded on June 30, 1998 by NASM8846, which carried the same scope and classification system forward under AIA/NAS maintenance.
- A related family of MS drawings — MS21209 (locking helical insert, CRES) and MS33537 (standard assembly dimensions) — call out MIL-I-8846 (now NASM8846) as their procurement specification.
- After installation, the driving tang on a self-locking or standard helical insert is typically broken off flush with the surface using a break-off tool.
Type & Class Reference
| Type | Thread Form | Available Classes |
|---|---|---|
| Type I | Coarse Thread | Class 1 (Free Running), Class 2 (Screw Locking) |
| Type II | Fine Thread | Class 1 (Free Running), Class 2 (Screw Locking) |
| Type III | Taper Pipe Thread | Class 5 (NPT), Class 6 (ANPT) |
| Type IV | Metric Spark Plug Thread | Class 3 (Staking) |
MIL-I-8846 is superseded. Current and future procurement should reference NASM8846, which carries the same scope, type, and class system forward under Aerospace Industries Association maintenance. This page is retained for legacy drawing cross-reference only.
MIL-I-8846 — Frequently Asked Questions
Is a "Heli-Coil" the same thing as a MIL-I-8846 insert?
Heli-Coil is a trade name commonly used for helical coil screw-thread inserts of this general type. MIL-I-8846 (now NASM8846) is the governing military specification that this style of insert is manufactured and qualified against.
What's the difference between free-running and screw-locking classes?
Free-running inserts provide a standard internal thread with no added resistance. Screw-locking (self-locking) inserts incorporate a feature that grips the mating fastener's threads to resist loosening under vibration.
Why are helical coil inserts used in soft materials?
Aluminum, magnesium, and other soft materials are prone to thread stripping under repeated torque or vibration. A helical coil insert transfers the load into hardened steel or CRES coil threads bonded into the tapped hole, dramatically improving thread durability without changing the base material.
Need Helical Coil Inserts to NASM8846?
Ananka Fasteners supplies current NASM8846 helical coil thread inserts with full material traceability.
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