Aerospace Threaded Inserts: Helical Coil and Key-Locking
Aerospace threaded inserts put steel threads into soft parent material. Helical coil inserts are bought to NASM21209 (screw-locking) and the NASM8846 procurement spec, with assembled hole dimensions per NASM33537; solid key-locking inserts are bought to MS51830 or MS51831 under MIL-I-45914 and installed per MS51835.
Updated
Insert types and where they are used
There are two families: wound wire helical coil inserts and solid bushings locked by keys. Both give a wear-resistant steel thread in aluminum, magnesium or plastic, and both are used for original design as well as repair.
| Type | Governing documents | Typical use |
|---|---|---|
| Helical coil, free running | NASM8846 procurement spec; inch free-running and screw-locking parts also listed under NAS1130 | Threads that need strength and wear life but no prevailing torque |
| Helical coil, screw-locking | NASM21209 (formerly MS21209), CRES, coarse and fine | Bolts and screws in vibration where a locking thread replaces a lock nut or wire |
| Key-locking, miniature and lightweight | MS51830 under MIL-I-45914 | Thin-wall solid inserts, studs and repairs where a coil cannot be used |
| Key-locking, heavy duty | MS51831 under MIL-I-45914 | Thicker walls and higher pull-out loads |
The FAA General handbook describes helical coil inserts as precision-formed coils of 18-8 stainless steel wire with a diamond-shaped cross section that form unified coarse or fine class 2B and 3B threads when assembled, and accept UNJ external threads. It notes they are used both to restore damaged threads and in the original design of missiles, aircraft engines and accessories, particularly where frequent assembly and disassembly is expected or a screw locking action is wanted.
The specifications that govern inserts
The helical coil documents moved from military to AIA NASM numbers; the key-locking documents are still DoD specifications maintained by DLA. Status below is as listed on EverySpec as of October 2026.
| Document | What it covers | Status |
|---|---|---|
| MIL-I-8846D | Inserts, screw thread, helical coil, procurement spec | Cancelled, superseded by NASM8846 |
| NASM8846 | Inch series helical coil insert requirements | Active AIA standard |
| MS21209E | Insert, screw thread, coarse and fine, screw locking, helical coil, CRES | Cancelled, superseded by NASM21209 |
| MS33537E | Helical coil insert, standard assembly dimensions (the tapped hole) | Cancelled, superseded by NASM33537 |
| NASM33537 | Assembled insert dimensions, inch coarse and fine | Active, January 2017 publication per GlobalSpec |
| MIL-I-45914A | Insert, screw thread, locked in, key locked, general specification (25 April 1991) | Listed without a supersession on EverySpec |
| MS51830F w/Amendment 1 | Key-locked insert, miniature and lightweight (14 November 2012) | Active |
| MS51831G w/Amendment 1 | Key-locked insert, heavy duty (20 November 2012) | Latest revision listed on EverySpec |
MS51830F states that the requirements for acquiring the insert are the specification sheet plus MIL-I-45914, and that installation shall be per MS51835. Put all three on the PO or the drawing.
The Heli-Coil manufacturer's standards list also references NASM122076 through NASM122275 for corrosion-resistant helical coil coarse thread inserts and NAS1130 for inch free-running and screw-locking inserts. Use those numbers only when your drawing calls them, and confirm the current revision with the document owner.
Have a part like this?Send it in under a minute. A person replies within 1 business day.Decoding real part numbers
Helical coil, MS21209C0410
Aircraft Spruce's listing for MS21209C0410 shows a 4-40 thread with a nominal diameter of 0.112 inch and a nominal length of 0.112 inch, so the trailing 10 means a length of 1.0 times the nominal diameter. The same listing gives material as stainless steel per NASM8846 and finish as plain, dyed red, or dry film lube (the L suffix) per AS5272. Distributors list C numbers for coarse threads and F numbers for fine threads in this series. Confirm the size code against the NASM21209 sheet before ordering a size you have not bought before.
| Segment | MS21209C0410 | Meaning, per the distributor listing |
|---|---|---|
| MS21209 | Standard | Screw-locking CRES helical coil, now NASM21209 |
| C | Thread series | Coarse thread series |
| 04 | Size | 4-40 |
| 10 | Length | 1.0 times nominal diameter, 0.112 inch |
| (L) | Optional suffix | Dry film lubricant per AS5272 |
Key-locking, MS51830CA106L
This example is taken directly from MS51830F. It is a lightweight key-locked insert in A286 CRES with a 0.250-20 UNJC-3B internal thread and an internal thread lock.
| Segment | Meaning per MS51830F |
|---|---|
| MS51830 | Basic sheet number |
| CA | A286 CRES. A dash here means 303 or 303Se CRES; A means 4140 or 8740 alloy steel |
| 106 | Dash number from Table I (coarse) or Table II (fine, 200 series) |
| L | Internal thread lock for aerospace use; LM for non-aerospace; blank for no lock |
The sheet also sets marking: 303 has no mark, A286 has one line on top, alloy steel has two parallel lines. Inserts with internal threads 0.250 and smaller carry two locking keys 180 degrees apart; 0.3125 and larger carry four keys 90 degrees apart.
Materials, finishes and the trade-offs
Pick the insert material for the thread's temperature, the parent metal and the bolt, then pick the lubricant for the locking feature.
- 18-8 CRES coil is the default helical insert, per the FAA handbook. It wears well against steel bolts and resists corrosion.
- MS51830 materials: 303 or 303Se CRES, A286 CRES heat treated to 140,000 psi minimum tensile, or 4140 or 8740 alloy steel heat treated to 160,000 psi minimum and 36 to 40 HRC. Keys are 302 CRES.
- Finishes on key inserts: CRES is passivated per AMS 2700; alloy steel is cadmium plated per AMS-QQ-P-416 Type II Class 3. Cadmium is restricted on many programs, so check the drawing before accepting plated alloy steel.
- Lubricant on locking threads: MS51830 requires dry film lube per AS5272 Type I for aerospace locking inserts and MIL-PRF-46010 for non-aerospace, and warns that MIL-PRF-46010 is lead free and not technically equivalent, so it must be validated before aerospace use.
- Galvanic pairing: a stainless insert in aluminum is a dissimilar-metal joint. Many drawings call for wet installation with primer or sealant; see surface finishing and aluminum alloys.
Self-locking versus free running is a design call. The FAA handbook notes inserts are chosen in part where a screw locking action is wanted; a free running insert needs the joint to be locked another way, such as safety wire, a self-locking nut on the far side or a locking feature on the bolt. Key-locking inserts can be ordered with a locking internal thread (the L suffix) or without one.
Installation and removal
Installation tooling is specific to the insert type and size, and it is often the cause of rejected parts. The FAA General handbook lays out the helical coil sequence:
- Drill out the damaged thread, or the new hole, to the specified minimum depth.
- Tap with the insert tap for the required nominal size; tap length must equal or exceed the requirement.
- Gauge the tapped hole with a helical insert thread gauge.
- Install with the proper tool so the top coil sits 1/4 to 1/2 turn below the surface.
- Break off the driving tang with the proper tool. Tangs are removed from all through holes; in blind holes they may be removed when there is enough depth below the tang.
To remove a coil, the handbook describes seating an extracting tool blade at 90 degrees to the edge of the insert, then turning left under pressure until the insert backs out; threads are not damaged if it is done correctly. It also states that these are not specific instructions and that the manufacturer's instructions must be followed.
For key-locking inserts, MS51830 points to MS51835 for installation, and the locking keys are part of the insert as delivered. Hole size, tap, installed depth and the tool that seats the keys all come from MS51835 and the insert maker's instructions, so name MS51835 on the drawing and buy the matching installation tools with the first order. Ask the manufacturer for its removal procedure before you plan rework, and order spare inserts for any part likely to be reworked.
Thread repair versus design use
A repair insert restores a stripped thread in an existing part and is usually covered by a repair manual or an MRB disposition. A design insert is on the drawing from the start, and the tapped hole is dimensioned to NASM33537 or the key insert sheet. Buy repair inserts against the repair document, not by matching the bolt size alone, because the oversize tap and hole depth differ.
What to put on the PO
- Full part number and revision, for example NASM21209C0410 with the finish suffix, or MS51830CA106L, plus NASM8846 or MIL-I-45914 as the procurement spec.
- Manufacturer and lot. Key inserts carry a source identification mark per MIL-I-45914; ask for the maker and lot on the cert.
- Lubricant callout where the locking feature depends on it, AS5272 Type I for aerospace.
- Plating restrictions, for example no cadmium, written on the PO if your program requires it.
- Tooling, if you need the installation and extraction tools bought with the inserts.
- DFARS specialty metals where a defense contract carries 252.225-7009; CRES, A286 and alloy steel inserts are covered metals. See DFARS specialty metals.
- Certificate of conformance to the part standard, and material certs if the drawing requires them. See certs and traceability.
Counterfeit and quality risks, and how to receive inserts
Inserts are small, look generic and are easy to substitute, so the cert and the marking carry most of the proof. AS6174 covers counterfeit materiel of any type.
- Check material marks on key-locking inserts against the code: no mark for 303, one line for A286, two lines for alloy steel.
- Count keys against the size rule in MS51830: two for 0.250 and smaller, four for 0.3125 and larger.
- Confirm thread and length on a sample, and the finish color or lube, against the part number.
- Reject commercial coils offered as MS or NASM parts without a manufacturer cert; free running and locking coils look similar until you count prevailing torque.
- Keep lots segregated by part number and finish.
See receiving inspection and counterfeit parts avoidance for the full process.
How we source threaded inserts
We do not stock inserts, publish prices or claim distributor status for any brand. We source inserts through authorized and franchised distributors or the manufacturer, flow down your spec, finish and lubricant requirements, and check marking and paperwork before shipment. Because inserts usually go into a machined part, we can buy them alongside CNC machining of the housing on one PO, have the shop install them with the right tooling, and deliver the assembled part with one cert package. We also kit inserts with bolts and screws, nuts and washers. Hard to find sizes and discontinued finishes go through our obsolete parts service. Related: rivets, pins, panel fasteners and safety wire, and the fasteners hub.
Get inserts quoted, installed or kitted
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Questions
What is the difference between NASM21209 and NASM8846?
NASM8846 is the procurement specification that sets general requirements for inch series helical coil inserts. NASM21209 is a part standard for a specific product: screw-locking CRES helical coil inserts in coarse and fine threads. A drawing typically calls the NASM21209 part number, and the supplier builds and tests it to NASM8846. Both replaced cancelled military documents, MS21209 and MIL-I-8846.
When should I use a key-locking insert instead of a helical coil?
Use whichever the drawing or repair document names; the two are not interchangeable. Key-locking inserts are solid threaded bushings held by locking keys, bought to MS51830 (miniature and lightweight) or MS51831 (heavy duty) under MIL-I-45914 and installed per MS51835. Helical coils are wound wire inserts bought to NASM8846 and its part standards. If a design is still open, the stress engineer chooses based on wall thickness, load and repair strategy.
Does the tang have to be removed from a helical coil insert?
In through holes, yes. The FAA General handbook says tangs are removed from all drilled through holes. In blind holes the tang may be removed when there is enough hole depth below the installed insert. Use the breakoff tool for that size, and retrieve the tang so it does not become foreign object debris inside an assembly.
Can a helical coil insert be removed and replaced?
Yes. An extracting tool blade is seated at 90 degrees to the edge of the insert and turned left under pressure until the coil backs out. The FAA handbook notes that the tapped threads are not damaged if the insert is removed properly. Install a new insert of the same part number afterward, and follow the manufacturer's instructions for the specific insert.
What does the L suffix mean on insert part numbers?
It depends on the standard. On MS51830 key-locking inserts, L adds an internal thread lock for aerospace use and LM the non-aerospace version. On MS21209 helical coils, a distributor listing shows L as the dry film lubricant finish per AS5272. Always read the suffix from the governing standard, not from another family.
Do threaded inserts fall under DFARS specialty metals?
They can. Stainless, A286 and alloy steel inserts are made of covered specialty metals, so if your defense contract carries DFARS 252.225-7009 the inserts must be from a compliant melt or fall under an exception named in the clause, such as the commercial fastener provisions. Put the requirement on the PO so the supplier can certify it.
Related
Sources
- FAA-H-8083-30A Aviation Maintenance Technician Handbook, General, helicoil inserts
- EverySpec, MS21209E superseded by NASM21209
- EverySpec, MIL-I-8846D superseded by NASM8846
- EverySpec, MS33537E superseded by NASM33537
- GlobalSpec, NASM33537 helical coil insert assembly dimensions
- EverySpec, MS51830F with Amendment 1, key-locked insert, miniature and lightweight
- EverySpec, MS51831G with Amendment 1, key-locked insert, heavy duty
- EverySpec, MIL-I-45914A key locked insert general specification
- Aircraft Spruce, MS21209C0410 listing (material, finish, dimensions)
- Stanley Engineered Fastening, Heli-Coil standards and specifications
- SAE AS6174, Counterfeit Materiel
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