MIL-I-8846 Helical Coil Inserts — Screw-Thread (Legacy)
The specification for MIL-I-8846 helical coil inserts covered screw-thread inserts made from formed wire, commonly recognized in the industry under the trade name Heli-Coil. Once installed, their inner surfaces provide robust threads. The final MIL-I-8846 revision D (8 March 1994) was superseded on June 30, 1998, by NASM8846, which serves as the active governing document today.
At a Glance
MIL-I-8846 Helical Coil Inserts Overview
MIL-I-8846 originally defined the standards for helical coil screw-thread inserts manufactured from precision-formed wire. When assembled into a tapped hole, the diamond cross-section of the wire expands outward, anchoring the coil in place while providing a finished, highly durable internal thread.
These components are intended primarily for original aerospace design applications—especially within soft base materials like aluminum and magnesium. By transferring structural loads from the soft metal to the high-strength stainless steel coil, they actively protect against thread failure caused by stripping and galling.
- The standard defined multiple categories, including coarse thread, fine thread, taper pipe thread, and metric spark plug configurations.
- The specification allowed for both free-running inserts and self-locking "screw-locking" variants that feature a deformed coil to grip the mating bolt.
- The final MIL-I-8846 revision D was formally superseded on June 30, 1998, transferring to the Aerospace Industries Association's maintenance as NASM8846.
Installation and Thread Repair
Beyond original equipment manufacturing (OEM), these components serve as the industry standard thread repair inserts. If a thread is stripped or damaged in the field, technicians can drill it out, tap it with an oversized specialized tap (often referred to as an STI tap), and thread the coil into place. 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 to allow full fastener pass-through. Current documentation for the tooling and military cross-references can be located on the Defense Logistics Agency (DLA) ASSIST portal.
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.
Frequently Asked Questions
Is a "Heli-Coil" the same thing as a MIL-I-8846 insert?
Heli-Coil is a well-known commercial trade name for helical coil screw-thread inserts. MIL-I-8846 (now NASM8846) is the formal governing military specification that guarantees this style of insert is manufactured and tested to aerospace standards.
What's the difference between free-running and screw-locking classes?
Free-running inserts provide a standard internal thread with no added installation resistance. Screw-locking (self-locking) inserts incorporate a distinct deformed coil that grips the mating fastener's threads, heavily resisting loosening under severe vibration.
Why are helical coil inserts used in soft materials?
Aluminum, magnesium, and plastics are highly prone to thread stripping under repeated torque or vibration. A helical coil insert transfers the load from the weak parent material into hardened steel or CRES coil threads, dramatically improving durability.
Need Helical Coil Inserts to NASM8846?
Ananka Fasteners supplies current NASM8846 helical coil thread inserts for aerospace and industrial repair, with full material traceability.
Prefer email? sales@anankafasteners.com
