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Threaded Inserts
A comprehensive engineering reference for mechanical design engineers, OEM manufacturers, aerospace and automotive teams — covering threaded insert types, installation methods, pull-out and torque-out capacity, substrate compatibility, dimensional standards and full project documentation for helical wire inserts, solid bushings, press-in inserts, heat-set and moulded-in variants.
Threaded Insert Types,
Installation Methods & Holding Mechanics
Aluminium · Plastic · CFRP · Timber · Sheet Metal
Definition and Engineering Function
A threaded insert is a fastener installed into a parent material to provide a strong, durable internal thread that is superior to a thread cut directly in the parent material. Threaded inserts serve three primary engineering purposes: (1) Thread reinforcement — providing a high-strength thread in a soft or weak parent material (aluminium, magnesium, plastics, timber) that cannot develop adequate thread strength by direct tapping; (2) Thread repair — restoring stripped or damaged threads in a parent material to full or greater-than-original strength without replacing the parent component; and (3) Thread conversion — enabling a different thread standard (e.g. metric in an inch-threaded housing, or a coarser thread from a finer original) to be used in an existing tapped hole.
The critical engineering advantage of a threaded insert is that it transfers the bolt load from the weak parent material thread-engagement zone to a larger bearing area on the insert body, which then distributes that load over a greater contact area in the parent material. A well-specified helical wire insert in aluminium, for example, develops the full proof load of a standard Grade 8.8 bolt — a load that would strip the soft aluminium thread by a wide margin if the bolt were threaded directly.
There are two independent failure modes for an inserted thread connection that must both be checked: (1) Thread strip — the internal thread of the insert strips under bolt torque load; and (2) Pull-out / torque-out — the insert body is pulled or rotated out of the parent material. The insert material and thread form control mode (1); the insert's external geometry, engagement length and parent material properties control mode (2). A well-designed insert system ensures that the bolt shank fractures (ductile steel failure) before either mode (1) or mode (2) occurs — this is the design objective for safety-critical threaded insert joints in aerospace and automotive applications.
Threaded Insert Types — Engineering Descriptions
Substrate Compatibility by Insert Type
Dimensions, Engagement Length
& Pull-Out and Torque-Out Capacity
Pull-Out · Torque-Out · 1.0D · 1.5D · 2.0D
Helical Wire Insert — STI Tap Sizes & Engagement Lengths
| Internal Thread | STI Tap Size (mm) | STI Drill Dia. (mm) | Insert OD (mm) | 1.0D Length (mm) | 1.5D Length (mm) | 2.0D Length (mm) |
|---|---|---|---|---|---|---|
| M3 | 3.6 | 3.4 | 3.6 | 3.0 | 4.5 | 6.0 |
| M4 | 4.7 | 4.5 | 4.7 | 4.0 | 6.0 | 8.0 |
| M5 | 5.9 | 5.6 | 5.9 | 5.0 | 7.5 | 10.0 |
| M6 | 7.0 | 6.7 | 7.0 | 6.0 | 9.0 | 12.0 |
| M8 | 9.3 | 9.0 | 9.3 | 8.0 | 12.0 | 16.0 |
| M10 | 11.5 | 11.2 | 11.5 | 10.0 | 15.0 | 20.0 |
| M12 | 13.9 | 13.5 | 13.9 | 12.0 | 18.0 | 24.0 |
| M16 | 18.4 | 17.9 | 18.4 | 16.0 | 24.0 | 32.0 |
| M20 | 22.8 | 22.3 | 22.8 | 20.0 | 30.0 | 40.0 |
Dimensions per NAS1130 / AS8879 / Heli-Coil standard. STI Tap size is the major diameter of the special Screw Thread Insert tap required — do NOT use a standard metric tap for helical wire insert installation holes. The STI tap creates a thread slightly larger than nominal to accommodate the insert body diameter. Insert length is expressed as a multiple of the nominal thread diameter (1.0D, 1.5D, 2.0D). 1.5D is the most common — providing full bolt proof load in aluminium and most soft metals. 2.0D recommended for high-load cyclic applications and materials weaker than Al 6061-T6.
| Substrate Material | Tensile Strength (MPa) | Pull-Out Force (kN) | Torque-Out (N·m) | Bolt Grade for Full Load |
|---|---|---|---|---|
| Al 6061-T6 | 310 | 12.5 | 28 | Grade 8.8 achievable |
| Al 5052-H32 | 228 | 9.2 | 20 | Grade 8.8 marginal — use 2.0D |
| Al 380 Die Cast | 324 | 13.0 | 30 | Grade 8.8 achievable |
| Low Carbon Steel | 400 | 28.0 | 55 | Grade 10.9 achievable |
| Nylon PA66 (30% GF) | 180 | 4.5 | 8 | Grade 4.6 only |
| ABS (unreinforced) | 45 | 1.2 | 2 | Low load only |
| CFRP UD Laminate | 600+ (0°) | 18.0* | — | *interlaminar shear governs |
Indicative values for SS 304 helical wire insert, 1.5D engagement, adhesive-free installation. Actual pull-out capacity depends on insert length, substrate alloy and heat treatment, installation quality (correctly torqued STI tap, clean hole), and whether locking adhesive (Loctite 638) was applied. For CFRP composites, interlaminar shear strength governs — not tensile strength — and specific testing on the actual laminate is required. All values are characteristic (without safety factor) — apply design safety factor of 2.0 minimum for safety-critical joints.
τ_substrate ≈ 0.577 × σ_y_substrate // Von Mises shear yield of substrate material [MPa]
// WORKED EXAMPLE: M8 Grade 8.8 bolt (F_proof = 22,100 N), Al 6061-T6 (σ_y = 276 MPa)
τ = 0.577 × 276 = 159 MPa
L_min = 22,100 / (π × 8 × 159) = 5.53 mm = 0.69D // Minimum for shear — specify 1.5D (12 mm) for margin
Material Grades, Substrate Compatibility
& Corrosion Considerations
Aluminium · Steel · CFRP · Thermoplastic · Timber
| Material | Standard | Hardness | Corrosion | Galvanic Risk in Al? | Best Application |
|---|---|---|---|---|---|
| SS 302/304 (1.4300) | ASTM A313 / AS8879 | ~35 HRC (spring) | High | Moderate | Standard helical wire; most industrial applications |
| SS 316 (A4) | EN 10151 | ~30 HRC | Very High | Moderate | Coastal, offshore, chemical, food-grade inserts |
| Phosphor Bronze (CuSn6) | BS 2873 PB101 | ~25 HRC | Good | Low | Marine Al structures; electronics; non-magnetic |
| Brass CuZn37 | DIN 17660 | ~15 HRC | Good | Low | Heat-set in plastics; press-in; non-sparking |
| Beryllium Copper (CuBe2) | ASTM B197 | ~40 HRC | Very High | Low | ATEX/non-sparking; aerospace; high strength |
| Titanium Gr.5 (Ti-6Al-4V) | ASTM F1472 | ~36 HRC | Extreme | Very Low | Aerospace structural; CFRP; weight-critical |
| Carbon Steel (hardened) | ISO 898-1 | 42–50 HRC | Low | High | Steel substrates only; not for aluminium or marine |
| Aluminium 6061-T6 | ASTM B209 | ~55 HRB | Good | Zero (matched) | Weight-saving self-tapping inserts in Al castings |
When a threaded insert is installed in aluminium or magnesium, the galvanic potential difference between the insert material and the substrate must be evaluated. Standard SS 304 helical wire inserts in aluminium have a moderate galvanic potential difference but the very small contact area of the wire coil (which limits corrosion current) and the protection of the enclosed installation environment make this combination acceptable for most applications with appropriate surface treatment. Carbon steel inserts in aluminium must never be used in any wet or outdoor environment — the galvanic couple accelerates corrosion of the aluminium at the insert boundary. For maximum galvanic compatibility in aluminium, specify aluminium inserts (self-tapping type) or phosphor bronze helical wire inserts. For CFRP substrates — which are electrically conductive and highly cathodic — titanium inserts are the only fully galvanically compatible metallic insert.
Selection Guide, Applications
& Quality Control and Documentation
Selection Matrix · Installation · QC · Documentation
Insert Selection Reference
| Substrate | Load Requirement | Installation Method | Recommended Insert Type | Material |
|---|---|---|---|---|
| Aluminium (structural) | Full Grade 8.8+ bolt load | Machine shop — drill + STI tap | Helical wire 1.5–2.0D | SS 304 or Phosphor Bronze |
| Aluminium (structural) | Very high / cyclic | Machine shop | Solid bushing (Keensert) | SS 304 or Ti Gr.5 |
| Aluminium (production) | Moderate | Press (production line) | Press-in knurled | SS 304 or Aluminium |
| Magnesium | Full bolt load | Drill + STI tap | Helical wire 2.0D | Phosphor Bronze (galvanic compat.) |
| Steel | Thread repair | Drill + STI tap | Helical wire 1.0–1.5D | SS 304 |
| CFRP / GFRP | Structural bolt load | Drill + STI tap + adhesive | Helical wire 2.0D | Titanium Gr.5 |
| Thermoplastic (ABS/PA/PC) | Moderate (production) | Heat-set / ultrasonic | Heat-set knurled insert | Brass |
| Thermoplastic (high volume) | Moderate (moulded) | In-mould (no secondary op.) | Moulded-in insert | Brass or SS |
| Timber / MDF / Plywood | Light–moderate | Hex driver or hammer | Self-tapping or barrel nut | Zinc alloy or SS |
| Sheet metal (<3 mm) | Pull-through retention | Press from rear | PEM / thinwall press-in | SS 304 or CS |
Applications by Industry
Helical wire inserts per AS8879 (formerly MIL-I-8846) are the standard thread reinforcement method throughout aerospace aluminium and magnesium structures, engine accessory gearboxes, avionics racks, fuel system brackets and landing gear components. Every tapped hole in an aerospace aluminium structural component that will accept a Grade 8.8 equivalent (NAS/AN bolt) fastener is typically STI tapped and fitted with a helical wire insert as standard design practice. The insert prevents thread strip, provides corrosion-resistant threads in bare aluminium, and allows the thread to be replaced (by removing and re-installing a new coil) if damaged during maintenance — without scrapping the parent component. Solid bushing inserts (Keensert, Kato) are used for the highest-load structural applications where helical wire pull-out strength is insufficient.
Helical wire inserts in cast aluminium engine blocks, cylinder heads, gearbox cases and differential housings allow assembly with standard steel bolts and studs at full torque without risk of thread strip. Magnesium engine components (used extensively for weight reduction in high-performance vehicles) require phosphor bronze or nickel alloy inserts to minimise galvanic corrosion — standard stainless inserts are marginally acceptable in dry protected locations but phosphor bronze is preferred for any component exposed to coolant, oil or road salt. Self-tapping inserts (Ensat type) are used in aluminium die-castings during the casting post-machining stage, offering fast installation without a separate tapping operation.
Heat-set brass inserts in ABS, PC/ABS, nylon PA66 and polypropylene enclosures are the standard method for providing re-usable bolt connections in injection-moulded electronic housings, instrument enclosures, control panel bezels, laptop and tablet cases, medical device housings and consumer appliances. The heat-set insert allows the enclosure to be opened and re-closed multiple times with the same bolt without degrading the thread — a direct thread in plastic would strip after 2–5 insertion cycles. The design rule for heat-set inserts is to specify the insert boss OD (moulded outer cylinder holding the insert) as minimum 2.0× the insert OD to prevent boss cracking under installation heat and bolt preload.
Carbon fibre reinforced polymer (CFRP) composites require special care for threaded connections because the fibres are easily damaged by direct tapping (which severs the fibres) and by conventional press-in inserts (which can cause delamination). Helical wire inserts installed with structural adhesive (Loctite 638 or equivalent) in STI-tapped composite holes provide the most reliable connection — the adhesive supplements the mechanical engagement and seals the hole against moisture ingress into the laminate. Titanium inserts are mandatory for structural CFRP applications where the galvanic potential of stainless steel combined with the highly cathodic CFRP surface would cause accelerated corrosion of any adjacent aluminium structure. Oversizing the hole by the STI tap amount slightly redistributes the stress concentration around the tapped hole in the laminate, reducing interlaminar shear stress at the hole edge.
Thread repair is the most economically critical application of helical wire inserts — a stripped threaded hole in an expensive casting, housing or structural member can be repaired to full or greater-than-original strength by drilling to the STI drill size, tapping with the STI tap, and installing the appropriate helical wire insert. The repair restores the original thread size and pitch, is invisible after installation, and does not require replacement of the parent component. The cost of a helical wire insert repair kit (drill, tap, inserts, installation tool) is a small fraction of the cost of a replacement casting or machined housing. Automotive workshops, aircraft MRO facilities, industrial maintenance teams and field service engineers routinely carry helical wire repair kits for M3–M16 and imperial equivalents.
Quality Control and Documentation
Threaded insert QC covers: (1) Dimensional inspection — wire diameter, coil OD (free state), internal thread gauge (Go/No-Go per ISO 1502), and insert length; (2) Material verification — PMI for stainless, phosphor bronze, titanium and exotic inserts; (3) Pull-out test — representative samples tested to failure in the specified substrate material with the specified engagement length — failure mode (pull-out vs thread strip vs bolt fracture) and failure load are recorded; (4) Torque-out test — resistance to rotation of the insert in the substrate under applied torque, critical for locking-type inserts (Keensert keys, locking coils); (5) Installation verification — for aerospace applications, STI tap inspection (tap wear, tap size gauge), correct installation tool and torque/position verification per the assembly procedure.
Export Packaging and Preservation
- Threaded inserts packed in polypropylene bags by type, thread size, length and material, labelled with standard reference, internal thread, insert length, material grade and batch/lot number
- Helical wire inserts particularly sensitive to tangles — do not pack loose in bulk bags; pack in individual counted trays or tube dispensers for production use
- Brass heat-set inserts and press-in inserts packed in component trays for automated assembly line use — orientation as specified by the customer tooling
- SS inserts in clean sealed bags; brass inserts wrapped or bagged to prevent tarnishing; carbon steel inserts with VCI for sea freight
- MTC (EN 10204 3.1), dimensional inspection report, thread gauge certificate, pull-out and torque-out test report (where required), PMI report for SS/Ti/BeCu, and all project documents in waterproof envelope with each shipment
| # | Document | Standard / Reference | Minimum Requirement |
|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 | 3.1 for OEM / industrial; AS9100D for aerospace |
| 02 | Dimensional Inspection Report | AS8879 / NAS1130 / DIN 16903 | Wire OD, free-state coil OD, insert length as mandatory chars |
| 03 | Thread Gauge Certificate | ISO 1502 / ASME B1.2 | Go/No-Go internal thread per lot; tolerance 4H6H confirmed |
| 04 | Pull-Out Test Report | AS8879 / MIL-I-8846 / project spec. | Failure load and mode in specified substrate material; required for structural |
| 05 | Torque-Out Test Report | AS8879 / project spec. | Required for locking inserts (Keensert, locking coil); torque-out value confirmed |
| 06 | PMI Report (XRF / OES) | Project specification | 100% of SS, phosphor bronze, Ti, BeCu and exotic material inserts |
| 07 | Hardness Test Report | ISO 6507 / ASTM E92 | Hardness range confirmed per material grade; required for spring-property inserts |
| 08 | ISO 9001 / AS9100D Certificate | ISO 9001:2015 / AS9100D | ISO 9001 for industrial; AS9100D required for aerospace supply |
| 09 | ISPM-15 Phytosanitary Certificate | IPPC / FAO | All wood packing for international export |
RR Hydraulics manufactures and exports threaded inserts in all types — helical wire (AS8879 / NAS1130), solid bushing (Keensert / Ensat), press-in knurled, heat-set, self-tapping and barrel nut — in stainless steel SS 302/304 and SS 316, brass CuZn37, phosphor bronze, beryllium copper, titanium Grade 5, and aluminium. Thread sizes M2–M36 metric and #2–1-1/2" inch UNC/UNF. EN 10204 3.1 MTC, thread gauge certificates, pull-out and torque-out test reports, PMI reports, AS9100D supply capability for aerospace. 48-hour express dispatch on standard in-stock sizes.