§1Threads in soft metal
Tapping a thread straight into aluminium, magnesium or plastic creates a mismatch: a hardened steel bolt is far stronger than the soft thread gripping it, so the joint fails by stripping the hole rather than breaking the bolt.
The bolt is not the weak part. As the metric fasteners page computes, an M10 class 8.8 bolt can carry 46.4 kN in tension before it breaks — but the internal thread that must hold it is only as strong as the material it is cut in, and cast aluminium has roughly a third of steel’s strength. The consequences are familiar to anyone who has worked on an alloy casting: threads that strip on first tightening, threads that will not take a proper preload, and threads that fail on the second or third assembly. There are two ways out. One is to make the hole very much deeper, so a longer thread shares the load (§2) — often impossible in a thin casting. The other is to put a steel thread into the soft metal, which is what a coil insert does (§3).
Contents§2How much engagement is needed
The rule that a thread should be engaged about one diameter deep is a steel rule. Work it out for a softer material and the required depth grows in proportion to how much weaker the material is.
The internal thread must shear before the bolt breaks, so its shear area times its shear strength has to beat the bolt’s 46.4 kN. Approximating the thread shear area as about 60% of the engaged cylinder, π × 10 × L × 0.6, and taking shear strength as roughly 0.6 of tensile: a mild steel nut at 400 N/mm² tensile shears at ≈240 N/mm², so one diameter of engagement (10 mm) carries π × 10 × 10 × 0.6 × 240 = 45.2 kN — almost exactly the bolt’s capacity. That is where the familiar 1D rule comes from; it is not arbitrary but the depth at which a steel thread and a steel bolt are equally matched. Now cast aluminium, at ≈150 N/mm² tensile and ≈90 N/mm² shear: the same 10 mm carries only 17.0 kN, barely a third. To reach the bolt’s full 46.4 kN takes 27.3 mm of engagement — 2.7 diameters. That is the whole problem in one number: a soft casting needs nearly three times the thread depth, and rarely has it.
§3What a coil insert is
A helical coil insert is a precision spring of hard, diamond-section wire that screws into an oversize tapped hole and presents a standard thread on its inside.
The wire has a rhomboid (diamond) cross-section, so when it is coiled, its outer edges form a thread that engages the oversize hole while its inner edges form a standard thread — M10 × 1.5, say — for the bolt (the hero). Wound from stainless or carbon steel, it is free-running and slightly springy, and it works by spreading the load: because the coil is resilient it deflects under load and distributes the tension along several turns rather than concentrating it on the first thread as a rigid thread does, so the soft parent carries the load over a larger area at lower stress. The result is a joint where the bolt is again the weakest link — which is what a designer wants, since a broken bolt is replaceable and a stripped casting often is not. A typical insert is 1.5 diameters long, and that modest depth in aluminium now does the work that 2.7 diameters of plain tapped thread would have needed.
Contents§4Fitting one
Fitting an insert is a four-step operation with one tool that must not be substituted — the oversize STI tap.
First drill the hole to the insert’s specified size, larger than the ordinary tapping drill for that thread since the finished hole must accommodate the coil’s outside. Then tap it with a Screw Thread Insert (STI) tap — an oversize tap made specifically for the purpose, cutting a thread of the same pitch but a larger diameter than the nominal. This is the step where jobs go wrong: an ordinary M10 tap cuts a hole the insert will not enter, and there is no substitute for the STI tap. Third, wind the insert in with its installation tool, which engages the small tang at the coil’s end, driving it until the top sits a quarter to half a turn below the surface. Finally, snap off the tang with a punch — it exists only to drive the coil, and if left in place it obstructs the bolt — and remove it from the hole. The insert is then a permanent part of the casting, and an ordinary M10 bolt threads straight into it.
Contents§5Repair and design-in
Coil inserts serve two quite different purposes — rescuing a stripped thread, and preventing one from ever forming — and the second is the more valuable.
As a repair, the insert is the standard answer to a stripped hole in a casting: drill out the ruined thread, tap oversize, wind in an insert, and the hole is restored to full size with a thread stronger than the original — a spark-plug hole in an alloy head, a stripped bolt hole in a gearbox case, a damaged thread in a machine frame. The alternative would be to scrap an expensive casting or to drill and tap a larger bolt, changing the design. As a design-in, the insert is specified from the outset wherever a steel fastener meets a soft parent — aluminium castings, magnesium housings, plastics — and especially where the joint will be repeatedly assembled and disassembled, since it is the fifth or tenth disassembly that pulls a plain aluminium thread out. It is the cheaper decision by a wide margin: an insert costs cents at design time and a scrapped casting costs a great deal at service time.
Contents§6Alternatives
The coil insert is not the only way to put a strong thread into soft or thin material — the alternatives suit different constraints.
Solid bushing inserts (key-locking types) are machined sleeves with an external thread and locking keys driven into the parent; they are more robust than a coil and better where the insert must resist being backed out, but they need more room. Press-in and self-clinching inserts are pressed into a plain hole rather than threaded, common in sheet metal and plastics. Moulded-in inserts are placed in the tool before a plastic part is moulded, so the plastic forms around them. And the plain engineering alternatives remain: use a through-bolt and nut so the load never touches a tapped thread at all — always the strongest answer where access allows — or simply make the boss deep enough to give the 2.7 diameters that §2 computed. The coil insert wins where the parent is soft, the space is tight, the joint is repeatedly disturbed, or a damaged thread must be salvaged. As with everything on this page, the choice comes back to the same question: what is the weakest link, and where do you want it to be?
Contents§7Quick reference
The working core of the page on one card rack.
The problem
steel bolt vs soft thread
hole strips before bolt breaks
Engagement
steel ≈ 1D · aluminium ≈ 2.7D
M10 8.8 needs 46.4 kN held
The insert
diamond-section wire coil
spreads load over turns
Fitting
drill → STI tap → wind → snap tang
STI tap is not optional
Use
repair stripped threads
design-in for soft parents