Aluminum, magnesium, and plastic all hold threads badly. Under repeated assembly, or against high preload, the internal threads gall, deform, or strip out entirely. The engineering answer is not "make the boss bigger" — it's to install a hardened steel thread inside the soft parent material. That's a threaded insert.

Three families dominate:

• Wire coil inserts (Heli-Coil): A precision-rolled diamond-shaped stainless wire, wound into a spring. You drill and tap the parent hole with a special oversized STI (Screw Thread Insert) tap, then thread the coil in with an installation tool. The coil expands slightly against the parent threads, gripping through friction and elastic preload. Cheap and repairable — the standard for stripped spark plug holes, aircraft aluminum, and cylinder heads.
• Solid bushing inserts (Time-Sert, Keensert): A hardened solid steel sleeve with external threads. Time-Serts have a thin skirt that a driver flares outward at the bottom of the hole, locking the insert mechanically against pull-out. Keenserts use small keys hammered into slots that cut into the parent metal. Stronger under vibration than wire coils, but the installed hole is larger.
• Self-tapping inserts (E-Z Lok, TAPPEX): Cut their own threads as they're installed. Great for plastic, wood, and low-volume aluminum work — no separate tap required.

Why the load actually distributes better: A bolt threaded directly into aluminum concentrates ~35% of the load on the first engaged thread. With a wire coil insert, the flexing coil deflects under load and shares the burden across all engaged threads more evenly — often giving higher pull-out strength than the parent material's native thread would provide.

Rule of thumb for insert length: Use an insert length equal to 1× the bolt diameter for steel screws into steel, 1.5× diameter for aluminum, and 2× diameter for magnesium or plastic. So an M8 bolt into an aluminum housing wants a 12 mm insert.

Real-world example: Aircraft engine cylinder heads use Heli-Coils on every spark plug hole from the factory — not as a repair, but as original equipment. The aluminum head can't survive 50,000 spark plug changes over the engine's life, and a wire coil insert lets a mechanic swap a plug with a torque wrench without eventually killing the head. Automotive OEMs increasingly do the same on transmission bellhousings and intake manifolds.

Installation gotcha: The tap drill for a wire coil insert is larger than a standard tap — an M6 Heli-Coil needs an M6 STI tap, which cuts an M7-ish hole. Using a standard M6 tap and trying to force the coil in will strip the coil, the parent, or both.