Guide
Do aluminum die castings need wire thread inserts, or will tapped threads hold?
Wire thread inserts are precision coils of diamond-section stainless steel wire, installed in an oversize tapped hole, that give a soft parent metal such as die-cast aluminum a hard, wear-resistant internal thread. Engineers often call them helicoils, after the Heli-Coil trademark, but the part itself is a generic fastener type with its own military and aerospace specifications.
By Saola Manufacturing engineering team · Published · Updated
The question usually follows a stripped boss or threads worn out after a few service removals. This guide covers why cast aluminum threads fail, how to check engagement, what an insert changes, which insert family fits, when another fix is better, and what to write on the drawing for an overseas supplier. The strength reasoning comes from two public NASA fastener documents; the casting guidance from NADCA.
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Why do tapped threads strip in aluminum die castings?
Tapped threads strip in cast aluminum because the steel screw is much stronger in shear than the aluminum around it, so the parent threads give way first unless the engagement is long enough to spread the load. Die castings add two aggravating factors: porosity in thick bosses and wear from repeated assembly.
A thread joint can fail by the screw breaking, the screw threads shearing, or the internal threads shearing. With a steel screw in cast aluminum, the third mode governs, because each engaged aluminum thread carries its share of the load across a thin cylindrical band of weaker metal.
Die casting then makes that band less predictable. NADCA’s design guidance lists internally tapped holes among the features that cannot be die cast, so the thread is made in a secondary operation (NADCA, secondary processing). Tapping cuts through the dense as-cast surface into the interior of the boss, which is where gas and shrinkage porosity typically collect in a heavy section. A void that falls on a few thread flanks removes their load-bearing area, and nothing in a visual inspection of the hole shows it.
Service is the other factor: aluminum threads wear and gall when a steel screw goes in and out repeatedly. NASA’s fastener manual describes an insert as a way to provide a strong, wear-resistant tapped hole in a soft material, and to repair stripped threads in an existing hole (NASA RP-1228, Fastener Design Manual).
How much thread engagement does a tapped hole in cast aluminum need?
Enough that the screw would break in tension before the aluminum threads strip. You can estimate that length from the pull-out formula in NASA’s fastener manual, using the published shear strength of the casting alloy.
NASA RP-1228 gives the pull-out load of a threaded hole as the pitch-diameter circumference of the hole multiplied by the length of engagement and by the shear strength of the parent material, divided by three. In symbols, P = π × dm × Fs × L / 3, where dm is the pitch diameter of the thread, Fs is the ultimate or yield shear stress of the weaker material, and L is the engaged length. The manual explains the divisor: with perfectly matched threads the cylindrical shear area would split evenly between the screw and the hole, giving a factor of one half, and the one-third factor is an empirical allowance for thread mismatch.
To size a tapped hole, set P equal to the tensile capacity of the screw and solve for L. Because Fs for a cast aluminum alloy is low compared with the tensile strength of a steel screw, the required L comes out several screw diameters long for a strong screw, and a cast boss rarely has that much depth to give. That is the numerical reason inserts exist.
Use the shear strength of the alloy the drawing actually names, such as its ASTM B85 designation, not a wrought figure; count only complete threads in L; and allow margin for porosity. NASA-STD-5020B, the NASA technical standard for threaded fastening systems, states the design goal plainly: engagement in an internally threaded part other than a nut or insert should be long enough that the fastener would fail in tension before the threads strip, because a broken screw is easier to detect and repair than a stripped hole (NASA-STD-5020B).
How does a wire thread insert make an aluminum thread stronger?
It moves the shear surface in the aluminum out to a larger diameter. The insert’s outer thread is bigger than the screw thread, so for the same engaged length the aluminum carries the load over a bigger cylinder, while the hard stainless coil takes the wear from the screw.
NASA RP-1228 illustrates the effect with a size example: the external thread of an insert is nominally one eighth of an inch larger in diameter than its internal thread, so a 10-32 screw in an insert behaves, as far as the parent metal is concerned, like a 5/16-18 thread in a nut (NASA RP-1228). Plugging that into the pull-out formula above, the parent shear area scales with the pitch diameter, so the gain for a small screw is substantial; for large screws the fixed one-eighth step is a smaller fraction of the diameter and the gain shrinks.
RP-1228 describes the coil as a precision coil of diamond-shaped stainless wire, made slightly oversize so that it springs out into an interference fit in the tapped hole, with a tang that is broken off at a notch after installation. The screw then runs against stainless steel instead of aluminum, which addresses the wear problem.
NASA-STD-5020B changes how such a joint is analyzed. An insert has two allowables, its internal-thread strength and its pull-out strength from the parent metal; use the lower, taken from the insert specification rather than a stripping calculation. Specifications usually compute pull-out as a minimum shear area times the parent metal’s ultimate shear stress, valid for solid, homogeneous material, and the standard says other parent materials need test data. A porous die-cast boss is not perfectly homogeneous, so pull-test a few inserts in production castings before release.
Which type of threaded insert suits a die-cast part?
For most die-cast housings a free-running or self-locking helical-coil insert is the default, because it needs the least extra hole size and is easy to replace. Heavy-duty key-locked solid inserts give more pull-out resistance than a helical coil of the same length, where the boss has room for them.
| Insert family | How it holds in the casting | Locking feature | Governing specification | Where it fits (editorial) |
|---|---|---|---|---|
| Helical coil (wire thread) | Oversize coil springs into a tapped hole with an interference fit | Free-running, or a deformed coil section that creates running torque | NASM 8846 | Repairs, smallest hole enlargement, easy replacement |
| Key locked-in solid insert | Keys on the outer diameter are driven in to stake it into the parent metal | Deformed or undersized portion of the internal thread | MIL-I-45914; examples MS 51830, MS 51831, MS 51832 | Heavy-duty and extra-heavy-duty types: more pull-out than a coil of the same length, where the boss has wall to spare |
| Thin-wall locked-in insert | Shoulder at the top is swaged into the parent metal with a special tool | Deformed internal thread | MIL-I-45932 | A solid insert locked by swaging instead of driven keys |
RP-1228 calls the wire thread insert the most popular choice for repairing a stripped hole because it needs the least hole enlargement, and says a solid bushing insert is preferred when space permits. NASA-STD-5020B agrees on the trade: helical coils are generally easier to replace than key-locked inserts, while heavy-duty key-locked inserts give more pull-out resistance than helical coils of the same length. It also warns that a helical coil installed incorrectly can back out under high vibration.
Two details from NASA-STD-5020B are worth copying into any design that puts stainless inserts in aluminum. The qualification test in NASM 8846 counts locking cycles with the screw unseated, so that figure should not be read as a seated-cycle life for a joint that is torqued down each time. And because a stainless coil in aluminum is a galvanic couple, common practice is to wet-install the coil with a corrosion-inhibiting primer at the insert and base-metal interface, keeping excess primer out of the screw thread where it would change the tightening torque.
When is longer engagement or a cast-in stud better than an insert?
When the boss already has depth, simply tapping deeper is the cheapest fix and adds no part to the bill of materials. A cast-in steel stud or insert suits joints that are assembled once and never opened, but it moves the risk into the die-casting process.
Longer engagement works when the pull-out calculation shows the needed length fits inside the boss with sound wall around it, and the joint is not opened often. It fails on the wear problem, because the screw still runs against aluminum. A cored pilot hole helps here: NADCA’s design guidance notes that engineers almost always put a hole in the center of a boss to keep wall thickness uniform (NADCA, bosses), and a boss with uniform walls is typically less prone to the internal shrinkage that undermines a tapped thread.
A cast-in stud or threaded insert is placed in the die before each shot and the aluminum solidifies around it. It avoids tapping, but adds a loading step and a presence check to every cycle. It is also hard to repair: if the stud is damaged, the casting is usually scrap, whereas a damaged helical coil can be extracted and replaced.
In short: plain tapped threads with verified engagement for joints assembled once; helical coils where the joint is opened in service or the boss is short; key-locked inserts for the highest pull-out where the boss has room; cast-in parts only for high-volume joints that never need repair.
How should thread inserts be called out for an overseas supplier?
Call out the insert by specification or part number, the installed thread, the insert length and the locking type, and state when it is installed relative to finishing. A supplier in Vietnam, Mexico or anywhere else can then quote the same part you had in mind.
- Name the insert. Give the procurement specification, such as NASM 8846 for self-locking helical coils, or the part-number standard your company uses, plus free-running or locking. “Heli-coil here” on a drawing leaves the length, material and locking style open.
- Give the installed thread. The finished thread size and class, such as 10-32 UNF-3B or M5x0.8-6H, is what the screw sees; the tapped hole for the insert is a different, special thread, so say which one the dimension refers to.
- State the length as a multiple of diameter. Insert length is normally expressed as a multiple of nominal diameter; take the available lengths from the insert specification you name, and make sure the boss depth allows the insert length plus the tap runout.
- Sequence it against finishing. Say whether the insert goes in before or after anodizing, plating or powder coating; installing after finishing keeps coating out of the insert thread, and installing before means the insert thread must be masked. The finishing trade-offs are covered in the finish selection guide.
- Ask for the checks. Gauge the installed thread with a go/no-go plug, confirm the tang is removed on through holes, and, for a new casting, ask for a pull-out test on a sample from the production die, which is the test NASA-STD-5020B points to when the parent metal is not homogeneous.
If the tapped hole will be machined in the casting, the tolerance guide explains which features follow casting tolerances and which follow machining tolerances, and the RFQ package guide lists what else to send with the drawing. Saola Manufacturing sources ADC12 aluminum and Zamak 3 zinc die castings from partner foundries and machines them on CNC milling at its lead CNC plant, so the cast boss and the tapped hole can be reviewed together, and the quote names the production site and origin; send the drawing for a quote and mark which holes need inserts.
Frequently asked
Is Heli-Coil a standard or a brand?
Heli-Coil is a trademark. The generic part is a helical-coil or wire thread insert, and self-locking versions are covered by NASM 8846. Specify the generic part and its specification on the drawing, and name a brand only if your approved-parts list requires it.
Can a stripped thread in a die casting be repaired with an insert?
Usually, yes. That is the use NASA RP-1228 calls most popular for wire thread inserts, because they need the least hole enlargement. Check first that the boss wall around the larger tapped hole is still sound, since porosity exposed by the new tap can limit the repair.
Should inserts go in before or after anodizing?
Installing after finishing keeps the coating from building up in the insert thread or interfering with the interference fit; installing before requires masking. Whatever you choose, write it on the drawing so the supplier does not have to guess the sequence.
Do inserts cause corrosion in aluminum?
A stainless coil in aluminum is a dissimilar-metal pair. NASA-STD-5020B notes it is common practice to wet-install helical coils in aluminum with a corrosion-inhibiting primer at the interface, which is worth specifying for parts exposed to moisture or salt.
Is a longer screw an alternative to an insert?
It raises pull-out strength if the boss has depth and sound metal, but the screw still runs against aluminum, so it does not fix wear on joints opened in service.
Sources
- Fastener Design Manual, NASA Reference Publication 1228, R. T. Barrett, NASA Lewis Research Center, 1990
- NASA-STD-5020B, Requirements for Threaded Fastening Systems in Spaceflight Hardware, NASA Office of the Chief Engineer, 2021 (revalidated January 2026)
- Secondary processing costs, NADCA Design, accessed September 2026
- Bosses, NADCA Design, accessed September 2026