Helical Coil vs Key-Locking Inserts: How to Choose
Choose a helical coil insert (NASM8846 family, such as NASM21209) for small screws in aluminum, tight bosses and easy replacement. Choose a key-locking insert (MS51830 or MS51831 under MIL-I-45914) for larger, heavily loaded or fracture critical bolts, sealed joints in corrosive service and repairs where the hole must not be reworked again.
Updated
The short answer, side by side
A helical coil is a wound wire spring held by its own outward pressure; a key-locking insert is a solid threaded bushing pinned into the parent metal by keys. Both put a steel thread into soft material. The part standards, decode examples and receiving checks are on the threaded inserts parts page; this page is about choosing between them.
| Factor | Helical coil insert | Key-locking insert |
|---|---|---|
| Construction | Diamond section CRES wire coil, slightly oversized for an interference fit, per NASA JSC | Solid bushing with locking keys, usually four on larger sizes, driven into the parent metal |
| Procurement documents | NASM8846 procurement spec; NASM21209 screw-locking parts; NASM33537 tapped hole | MIL-I-45914 general spec; MS51830 lightweight, MS51831 heavy duty; MS51835 installation |
| Hole preparation | Drill and special STI (screw thread insert) tap, STI gage | Standard drill, countersink and standard Unified tap, per Howmet's Keenserts instructions |
| Locking options | Free running or screw-locking; running torque per NASM8846 Table II | With or without internal thread lock; running torque per MIL-I-45914 Table 1 |
| Space needed | Smallest boss; Heli-Coil states a boss of twice the nominal bolt size is adequate for most loads | Larger hole and higher edge distance than a coil, per NASA JSC |
| Removal | Extracting tool turned counterclockwise; tangless versions allow nondestructive removal | Drill between keys, collapse and break keys, extract; same size insert reinstalled in the original hole |
| NASA JSC 2004 guidance | Best for #10 and smaller screws under small tension loads, in aluminum, at least 1.5D long | Recommended for larger, heavy duty tension bolts and fracture critical bolts |
Pull-out and thread strength: what the sources actually say
There is no single published head to head pull-out table that settles this; each maker publishes data that favors its own design, and the design rule both follow is to size the insert so the bolt breaks before the parent thread strips.
- Design to bolt failure. Heli-Coil Technical Bulletin 68-2 sets insert length by balancing the bolt's tensile strength against the shear strength of the parent material, and says it is good practice to choose an insert length whose assembly strength exceeds the screw's, so an overtorqued screw fails first. KATO's insert assembly article makes the same point: the bolt should be the weakest part.
- A worked number. TB 68-2 gives an example for a 2-56 thread in 6061-T6 aluminum of 30,000 psi shear strength: insert assembly strength of 470 lb at 1 diameter length, 810 lb at 1.5 diameters and 1,120 lb at 2 diameters, and concludes a 1.5D insert with a 170,000 psi screw supports a 500 lb load. These are chart values from one manufacturer, and the bulletin notes its shear values are averages at room temperature.
- The manufacturer claims differ. TB 68-2 states that wire inserts and solid bushings are essentially comparable at equal length and that, for most sizes, the wire insert has superior tensile strength because solid bushings need rings or keys that enlarge their outside diameter. Howmet, which makes Keenserts key-locking inserts, states its insert threads are stronger than original threads. Neither statement is an allowable for your joint.
- NASA JSC practice. A 2004 JSC Materials Branch presentation on fastener retention recommends helical coils at least 1.5D long, warns that shorter coils can shear threads and back out, recommends key-locked inserts for larger heavy duty tension bolts and fracture critical bolts, and states helical coils are not recommended for fracture critical single point failure bolts.
- Temperature. TB 68-2 warns that the shear strength of most light metals drops significantly at elevated temperature, so room temperature charts overstate hot strength. KATO likewise notes its table values are average calculated values, not guarantees.
For a flight joint, the stress engineer should set the insert length and type from the maker's data for the actual size, parent alloy, temper and temperature, and test where the margin is thin.
Have a part like this?Send it in under a minute. A person replies within 1 business day.Parent material and corrosion
Helical coils are at their best in aluminum; key-locking inserts handle sealed joints in corrosive service with less fuss.
- Aluminum. NASA JSC recommends helical coils in aluminum structure, noting the coil expands better and is held by friction there, and reports problems installing coils in steel structure, where Heli-Coil recommends a special TiN coated tap.
- Soft and cast alloys. Lower shear strength parent material needs a longer insert for the same bolt. TB 68-2 shows the 2-56 example in 356-T6 casting alloy of 26,000 psi shear, where the 1D insert at 400 lb cannot carry the 460 lb screw rating and the 1.5D insert at 700 lb can.
- Sealant and corrosion. NASA JSC calls corrosion protection sealant on coils tricky: too much flows through the coils and upsets running torque, too little does not protect. Solid key-locking inserts take sealant more predictably, so JSC sees no problem using them in severe corrosive environments with the right sealant.
- Galvanic pairing. Either insert puts stainless or alloy steel into aluminum. Treat wet installation and the hole finish as part of the design; the aluminum alloys page covers alloy and temper choices that set the parent shear strength.
Installation and removal tooling
Helical coils need size specific STI taps and inserting tools; key-locking inserts use a standard drill and tap but need the matching key drive tool. Plan tooling with the first insert order.
| Step | Helical coil (Heli-Coil catalog HC-2000) | Key-locking (Howmet Keenserts instructions) |
|---|---|---|
| Drill | Drill size from the catalog or NASM33537 tables | Standard drill listed for each part number |
| Countersink | Recommended; KATO cites 120 degrees under NASM33537 | Standard countersink, 82 to 100 degrees |
| Tap | STI tap, Class 2B or 3B; Class 3B recommended for screw-lock | Standard Unified thread tap |
| Gage | STI gage; locking inserts themselves cannot be gaged, per KATO | Standard thread gage for the tapped hole |
| Install | Inserting tool, hand or power, set below the surface | Screw in by hand or tool until the keys stop it at depth, then drive keys with the installation tool by hammer or arbor press |
| Finish | Break off and retrieve the tang, or use a tangless insert | No tang; keys set the depth automatically |
| Remove | Extracting tool blade struck lightly into the coil, then turned counterclockwise | Drill between keys, deflect keys inward and break off, remove with an E-Z OUT type tool |
Tangs are a foreign object debris risk. NASA JSC cautions that tangs must not be left inside hardware, and recommends blind holes over through holes for coils, with the back side inspected after each fastener installation when a through hole is used. Heli-Coil's tangless inserts remove the break-off step and allow nondestructive removal, which is worth specifying in assemblies where retrieving a tang is hard.
For military standard key-locking inserts, MS51830F directs installation per MS51835, so the drill, tap and tool come from that standard and the insert maker's instructions, not from a commercial catalog for a different product line.
Space, wall thickness and edge distance
Helical coils fit where there is the least metal around the hole; key-locking inserts need more.
- Boss size. The Heli-Coil design catalog states a boss diameter of twice the nominal bolt size is adequate for most load conditions, and for critical applications recommends twice the Heli-Coil tap major diameter. It also states coils can often be added to existing designs without increasing boss size.
- Material thickness. For a through hole, the catalog sets the minimum material thickness equal to the insert nominal length, without a countersink.
- Key-locking edge distance. NASA JSC notes that key-locked inserts require higher edge distance, and Howmet asks customers to consult its applications engineers for critical edge distance applications. Get the minimum edge and wall figures for the exact part number before the drawing is released.
- Thin wall solid inserts. NASA JSC describes a third option: solid thin wall inserts without keys, swaged into the parent metal, procured to MIL-I-45932, which it says are lighter than key-locked inserts and suit joints with less edge distance. Confirm the current status of that document on ASSIST before designing one in.
Within the key-locking family, the series matters too: MS51830 is the miniature and lightweight sheet and MS51831 the heavy duty sheet, so check the drill size on the sheet for the series you choose before fixing the boss.
Repair versus design use
Both types are used in original designs and for thread repair; the difference is how often the hole can be repaired again.
- Helical coil repair. Heli-Coil thread repair kits contain inserts, the matching drill, an STI tap and an installation tool, and the professional kits add a tang removal tool and three insert lengths. NASA JSC notes coils are relatively easier to replace and reinstall than key-locked inserts.
- Key-locking repair. Howmet states that after removal an identical insert goes back into the original hole with no rework, because the keys are broken out rather than the tapped thread being disturbed.
- Repair documents rule. On fielded hardware, the repair manual or the material review disposition names the insert. Buy to that document, not to the bolt size, because the tapped hole differs between insert types and between makers.
- Design use. For new parts the tapped hole is dimensioned to NASM33537 for coils or to the key-locking insert sheet, and the insert is listed on the drawing or bill of materials.
Locking options
Both types come in locking versions, and both need running torque checked because the locking feature wears.
- Screw-locking coils. One or more deformed coils grip the screw. The Heli-Coil catalog notes inch screw-locking inserts are dyed red per NASM21209 and that locking torque meets NASM8846 Tables I and II. NASA JSC says there is no fixed cycle life: the only way to know the lock has worn out is to measure running torque at every installation against NASM8846.
- Lubrication. NASA JSC recommends dry film lubricated locking coils, the L suffix on NASM21209 part numbers, to avoid binding. The Heli-Coil catalog requires anti-seize on heat treated steel or stainless screws used in screw-locking inserts. See dry film lubricants.
- Key-locking inserts. MS51830 inserts can be ordered with an internal thread lock, the L suffix for aerospace, with running torque per MIL-I-45914 Table 1.
- Patched screws. NASA JSC advises against plastic patch screws in helical coils in fracture critical applications because of binding, and says patches can be used with solid inserts.
- Preload sensitive joints. JSC recommends caution with coils in joints sensitive to gapping, because high preload can bind poorly lubricated threads.
Specification families
The coil documents moved to NASM numbers; the key-locking documents are still DoD documents. Status below is as listed on EverySpec and the Heli-Coil standards list in October 2026; confirm the current revision on ASSIST or SAE before ordering.
| Document | Covers | Status |
|---|---|---|
| NASM8846 | Helical coil insert procurement requirements, inch series | Active; supersedes MIL-I-8846 |
| NASM21209 | Screw-locking CRES helical coil inserts, coarse and fine | Active; supersedes MS21209 |
| NASM33537 | Helical coil assembled insert and tapped hole dimensions | Active; supersedes MS33537 |
| NASM122076 thru NASM122275 | Free running CRES helical coil inserts, coarse thread | Listed in Heli-Coil's standards list |
| MIL-I-45914A | Key locked insert general specification | Listed on EverySpec without supersession |
| MS51830F w/Amendment 1 | Key-locked insert, miniature and lightweight | Active |
| MS51831G w/Amendment 1 | Key-locked insert, heavy duty | Latest listed revision |
| MS51835 | Installation of key-locked inserts, named by MS51830F | Confirm the current revision on ASSIST |
Part number decodes for NASM21209 and MS51830 are worked through on the threaded inserts page.
Choose helical coil when, choose key-locking when
| Choose a helical coil insert when | Choose a key-locking insert when |
|---|---|
| Screws are #10 and smaller under modest tension, per NASA JSC guidance | Bolts are larger or carry heavy structural tension |
| The parent is aluminum and the boss or edge distance is tight | There is room for the larger hole and edge distance the keys need |
| Weight and envelope are driving the design | The bolt is fracture critical, where JSC recommends key-locked inserts |
| Field replacement with a simple extractor is expected | The joint is sealed for severe corrosion and sealant must not upset running torque |
| A drawing, repair manual or NASM part number already calls a coil | A repair must be repeatable in the same hole without reworking the thread |
Cost and kit considerations
Piece price is rarely the deciding cost; tooling, installation labor and rework risk are. No prices are given here because they vary with size, material, finish and quantity.
- Tooling. Coils need an STI tap, gage and inserting tool for each size, and a tang break-off tool unless tangless inserts are used. Key-locking inserts use standard drills and taps but need the installation tool for each insert type Howmet also lists broaching tools coded by tap drill diameter and key count for some installations.
- Insert cost. A key-locking insert is a machined bushing with assembled keys, while a coil is formed wire. Heli-Coil's catalog claims lower insert and installation cost for coils; treat that as a manufacturer's statement and compare installed cost for your part.
- Labor and escapes. Coil installation adds tang retrieval and back side inspection on through holes. Key-locking installation adds key driving with the right tool and a check that the insert sits at the depth the keys set.
- Kits. Order taps, gages and tools with the first insert lot, and keep spare inserts for any part likely to see rework. We build kits that pair inserts with the bolts and screws they receive.
- Availability. Specific finishes, lubricants and material codes can be hard to find in small lots. Our obsolete parts process covers discontinued variants under counterfeit avoidance controls.
Callout examples
Example formats only. Engineering selects the part number, length and finish, and confirms the current revision on ASSIST or SAE.
- Helical coil, design: INSTALL HELICAL COIL INSERT NASM21209[size and length code]L. TAPPED HOLE PER NASM33537. REMOVE TANG AND RETRIEVE. VERIFY RUNNING TORQUE PER NASM8846.
- Key-locking, design: INSTALL KEY-LOCKED INSERT MS51830CA106L PER MS51835. This MS51830 part number is the example given in MS51830F: A286 CRES, 0.250-20 UNJC-3B, internal thread lock.
- Repair: REPAIR STRIPPED THREAD PER [repair document and paragraph], INSERT [part number named in repair document]. INSTALL WET WITH [sealant or primer per drawing].
What to put on the PO
- Full insert part number with length and finish or lubricant suffix, plus NASM8846 or MIL-I-45914 as the procurement spec
- Installation standard, NASM33537 or MS51835, if inserts will be installed by a supplier
- Manufacturer and lot on the certificate of conformance
- Plating restrictions, for example no cadmium, if your program requires it
- Tooling to be bought with the inserts: taps, gages, inserting, tang break-off, key drive and extracting tools
- DFARS 252.225-7009 specialty metals flow down where the contract carries it; see DFARS specialty metals
- Running torque verification records where the drawing requires them
Get inserts sourced, installed or kitted
Send the insert part numbers, the drawing or repair document that calls them and the part they go into through the hardware request form. We source inserts and tooling through authorized and franchised distributors or the manufacturer, and if the housing is being machined we can quote CNC machining and installation on one PO with one cert package. A person replies within one business day. More process references are in the processes library.
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Questions
Which is stronger, a helical coil or a key-locking insert?
It depends on length, size and parent material, and the manufacturers disagree. Heli-Coil Technical Bulletin 68-2 states wire inserts and solid bushings are essentially comparable at equal length, with wire inserts stronger in most sizes. Howmet states its Keenserts threads are stronger than original threads. NASA JSC guidance from 2004 recommends key-locked inserts for larger heavy duty tension bolts and fracture critical bolts. Size the insert so the bolt fails first, using the maker's data.
How long should a helical coil insert be?
Long enough that the assembly is stronger than the screw. Heli-Coil sells 1, 1.5, 2, 2.5 and 3 diameter lengths and sizes them from parent shear strength and bolt strength. In its 2-56 example in 6061-T6, assembly strength rises from 470 lb at 1D to 810 lb at 1.5D and 1,120 lb at 2D. NASA JSC guidance says coils should be at least 1.5D to avoid thread shear and back-out.
Can I replace a helical coil with a key-locking insert in the same hole?
Not directly. A coil sits in an STI tapped hole sized for the coil, while a key-locking insert needs its own drill, standard tap and key engagement, and generally a larger hole and more edge distance. Changing types is a design or repair decision that needs the repair document or engineering approval, plus a check that wall thickness and edge distance still meet the key-locking insert maker's requirements.
Do key-locking inserts need special taps?
Not for the Keenserts product line. Howmet's installation instructions call for a standard drill listed for each part number, a standard 82 to 100 degree countersink and a standard Unified thread tap, then an installation tool to drive the keys. Military standard MS51830 inserts are installed per MS51835, so take tooling from that standard and the insert maker. Helical coils, in contrast, need size specific STI taps and gages.
How is a key-locking insert removed?
Howmet's procedure is to drill out insert material between the keys with the listed drill, deflect the keys inward and break them off, then back the insert out with an E-Z OUT type tool. It states an identical insert can then go into the original hole without reworking it. Plan rework with the maker's instructions for your exact part number, and control the chips as foreign object debris.
Why do helical coil inserts have a tang, and must it come off?
The tang gives the inserting tool something to drive. NASA JSC cautions that broken tangs must not be left inside hardware, and the FAA handbook says tangs are removed from through holes. Heli-Coil tangless inserts drop the break-off step and allow nondestructive removal, which helps in assemblies where retrieving a tang is difficult or where foreign object debris control is strict.
Do locking inserts wear out?
Yes, the locking feature loses running torque with use. NASA JSC states there is no fixed cycle life for locking helical coils and that the only way to know is to measure running torque at every installation against NASM8846. Key-locking inserts with internal locks have running torque values in MIL-I-45914 Table 1. Record the torque where the drawing requires it and replace inserts that fall below the minimum.
Related
Sources
- NASA JSC, Fastener Retention Requirements and Practices in Spaceflight Hardware (R. Dasgupta, 2004), NTRS 20100042553
- Stanley Engineered Fastening, Heli-Coil Technical Bulletin 68-2, Tensile Strength of Threaded Insert Assembly
- Stanley Engineered Fastening, Heli-Coil design catalog HC-2000 Rev 12
- Howmet Fastening Systems, Keenserts Inserts Installation, Removal and Broach
- KATO Fastening, KATOpedia insert assembly strength
- KATO Fastening, how to install thread inserts
- 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
- EverySpec, MS21209E superseded by NASM21209
- EverySpec, MIL-I-8846D superseded by NASM8846
- FAA-H-8083-30A Aviation Maintenance Technician Handbook, General
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