Threaded Inserts for Aircraft: Types, Part Numbers, and Installation

A threaded insert is a part installed in a prepared hole to provide internal threads for a screw or bolt. The three main types in aerospace are helical coil inserts, which are torqued into a tapped hole [1], key-locked inserts, which are held by driven keys [2], and potted inserts, which are bonded into honeycomb sandwich panels [4].

WHAT'S IN STOCK

Monroe's inserts category covers all three types: MS21209, NAS1130, and MS124817 helical coils; MS51830, MS51831, MS51832, and NAS1394 key-locked styles; and NAS1832 through NAS1836 potted styles, with NAS1837 installation tabs.

HELICAL COIL INSERTS

Helical coil inserts are made from a coil of diamond-shaped wire that forms both the internal and external threads [1]. In their free state, they're larger than the tapped hole. They're torqued into place, and when the torque is released the coils expand to create an interference fit with the tapped threads [1].

Types and part numbers, per NASA's installation spec [1]:

Locking - MS21209
Free-running - MS122076 through MS124850 ranges (MS124817 falls in this group)
Tangless - NAS1130

Most have a tang used to drive them in. After installation, the tang is broken off and removed [1]. Tangless inserts need special installation tooling [1].

Installation steps (NASA PRC-9008) [1]:

- Tap the hole with the matching STI tap. STI taps come in Class 2B and 3B, and the tap's class sets the class of the finished thread [1].
- Countersink the hole to the standard dimensions in NASM33537 unless the drawing says otherwise. The countersink also affects how deep the insert sits [1].
- For aluminum and dissimilar metal joints, wet installation with corrosion-preventive primer is standard [1].
- Remove the tang [1].
- Verify the correct STI tap, hole minor diameter, countersink, and thread form before installing, and check depth and tang count afterward [1].

Removal: Back it out with the manufacturer's tooling, re-inspect the hole, and install a new one. NASA's spec says helical inserts must not be reinstalled [1].

KEY-LOCKED INSERTS

Key-locked inserts are solid threaded bushings with keys that are driven into the parent material to stop rotation. NASA's key-retained spec covers MS51830 (miniature and lightweight), MS51831 (heavy duty), MS51832 (extra heavy duty), and NAS1394 (lightweight) and NAS1395 (heavy duty) [2].

How to read an MS51830 part number [3]:

MS51830 - Basic number.
Material code - No letter = 303 stainless steel. CA = A286 stainless steel. A = alloy steel.
Dash number - Internal and external thread size. For example, -106 is a .250-20 internal thread with a .375-16 external thread.
L suffix - Internal thread locking feature.

Example: MS51830CA106L is an A286 lightweight part with a .250-20 internal thread and thread lock [3].

Sizes with .250 and smaller internal threads come with two locking keys, and .3125 and larger come with four [3]. The standard says installation follows MS51835 [3].

Installation steps (NASA PRC-9006) [2]:

- Drill and tap the hole to the published size and depth, with UN threads to Class 2B (except where noted as NS) [2].
- Countersink the edge of the hole at 100° inclusive [2].
- Set it below the surface: miniature sizes 0.005 to 0.015 in. below; lightweight, heavy-duty, and extra heavy-duty 0.010 to 0.030 in. below [2].
- Drive the keys flush with or below the surface, using the manufacturer's tooling [2].
- In materials over 80,000 psi ultimate tensile strength, broach the key slots [2].
- Wet installation with corrosion-preventive primer is standard for aluminum and dissimilar metals [2].

Removal (NASA PRC-9006) [2]: Drill out the material between the keys, deflect the keys inward with a punch to break them off, and remove the insert with an E-Z Out type tool. If new key slots are needed, locate them about 90° from the originals for two-key styles or 45° for four-key styles [2].

POTTED INSERTS FOR HONEYCOMB PANELS

Potted inserts are bonded into honeycomb sandwich panels with an adhesive potting compound. Clarendon Specialty Fasteners notes they're widely used in aircraft walls and stowage bins, and anywhere a blind or flush attachment point is needed in a panel [4].

NAS potted types [4]:

NAS1832 - Blind
NAS1833 - Through
NAS1834 - Through, clearance hole and countersink
NAS1835 - Floating
NAS1836 - Blind, lightweight

Installation steps [4]:

1. Drill the panel to the insert diameter.
2. Enlarge the core with an undercutting tool. At least 0.040 in. of clearance around and below the insert is recommended so adhesive can flow.
3. Inject potting compound through the holes in the installation tab until it comes out the other side.
4. Remove the tab after the compound cures. The part sits flush with the surface.

For blind styles, choose a length that doesn't touch the inner surface of the bottom skin [4].

OTHER WAYS TO ADD THREADS

There are other ways to add threads. The FAA handbook describes Rivnuts as hollow, internally threaded blind rivets installed from one side, used mainly as nutplates and to attach deicer boots [5]. Nutplates are another common choice where you need a fixed nut behind a panel. Compare nutplate styles in our aircraft nuts guide, or shop nutplates. For thread series and fit classes, see our threaded fasteners guide. For damaged fastener holes rather than damaged threads, the FAA handbook describes Acres fastener sleeves. They're installed in holes drilled 1/64 in. oversize so the original-diameter fastener can be reused [6].

FAQ

Q: What's the difference between MS21209 and NAS1130 inserts?
A: Both are helical coil inserts. NASA's spec lists MS21209 as the standard locking insert and NAS1130 as the tangless insert [1].

Q: Can a helical coil insert be reused after removal?
A: No. NASA PRC-9008 says helical inserts must not be reinstalled [1].

Q: Do I have to remove the tang?
A: Yes, for tanged inserts. The tang is broken off and removed after installation [1].

Q: How deep should a key-locked insert sit?
A: Per NASA PRC-9006, 0.005 to 0.015 in. below the surface for miniature inserts and 0.010 to 0.030 in. for the others, with keys flush or below [2].

Q: What does the L mean in MS51830CA106L?
A: L adds an internal thread locking feature. CA means A286 material [3].

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Related guides: Threaded Fasteners | Aircraft Nuts | Aircraft Screws

This guide is part of the Monroe Aerospace Guides library. Browse the full library, or contact us if you need help matching an insert to a thread size.

References

[1] NASA JSC, PRC-9008 Rev E, Process Specification for Installation of Helical Coil Inserts: https://www.nasa.gov/wp-content/uploads/2023/03/prc-9008-current.pdf
[2] NASA JSC, PRC-9006 Rev F, Process Specification for the Installation of Key-retained Screw Thread Inserts: https://www.nasa.gov/wp-content/uploads/2026/07/prc-9006-rev-f.pdf
[3] MS51830F, Insert, Screw-Thread, Locked In, Key-Locked, Miniature and Lightweight: https://www.acmeindustrial.com/wp-content/uploads/2019/12/MS51830F.pdf
[4] Clarendon Specialty Fasteners, Potted Inserts: https://www.clarendonsf.com/usercontent/doc/18235/potted%20inserts_v2%2011.25_lr.pdf
[5] FAA, Aviation Maintenance Technician Handbook - General, Chapter 5 (Nutplates and Rivnuts), via AvStop: http://avstop.com/ac/Aviation_Maintenance_Technician_Handbook_General/5-71.html
[6] FAA, Aviation Maintenance Technician Handbook - General, Chapter 5 (Acres Fastener Sleeves), via AvStop: http://avstop.com/ac/Aviation_Maintenance_Technician_Handbook_General/5-74.html