Thread Repair Inserts: HeliCoil Installation and Selection

When to Use Inserts vs Oversizing vs Welding

Thread repair inserts are the right choice when you need to maintain the original thread size, when the hole location cannot change, when the parent material is softer than the bolt (aluminum, magnesium, cast iron, plastic), or when the repair must be as strong or stronger than the original thread. Inserts are also used proactively in new designs. Aluminum engine blocks, aircraft structures, and plastic housings use inserts at factory to provide durable threads in soft parent materials.

Oversizing (drill and retap) works when a larger bolt is acceptable, when the mating part can accommodate the new bolt size, and when the parent material has sufficient cross-section to support the larger hole. Oversizing is simpler than insert installation (no special tooling) but changes the bolt size, which may require modifying the mating part, the wrench used for service, and the torque specification.

Weld filling and retapping is used when the parent material is weldable, when inserts are not available for the thread size, or when the damage extends beyond what an insert can repair (e.g., a completely blown-out hole or cracked boss). The hole is welded full, the weld is machined flat, and the hole is drilled and tapped to the original size. This approach requires welding skill and may require post-weld heat treatment depending on the material.

Insert Types: Wire, Solid, and Key-Locked

Wire thread inserts (HeliCoil type): A coil of diamond-shaped stainless steel wire that is wound into a pre-tapped oversize hole. The insert provides a hardened thread surface while maintaining the original thread size and class. Wire inserts are the most common type for general repair and are available for virtually every standard thread size. They are thin-walled and add minimal material to the hole, making them suitable for thin bosses and tight spaces.

Solid inserts (threaded bushings): A cylindrical sleeve with external threads (to match the oversize tapped hole) and internal threads (at the original size). Solid inserts are stronger than wire inserts and are better suited for high-load applications, repeated assembly/disassembly, and situations where the insert must resist pullout from the parent material. They require a larger oversize hole than wire inserts, which means more material removal from the parent part.

Key-locked inserts (Keensert, KeySert): Solid inserts with locking keys that expand into the parent material after installation, preventing the insert from rotating or pulling out under load. Key-locked inserts are the highest-strength repair option and are specified for critical aerospace, military, and industrial applications where insert retention is a safety concern. They require specific installation tooling and a carefully prepared hole.

For general shop repair work, wire thread inserts handle 90% of applications. Solid inserts are the step-up when wire inserts are not strong enough. Key-locked inserts are for critical applications where failure of the insert itself would be dangerous.

The Drill-Tap-Install Process Step by Step

Step 1: Drill. Drill out the damaged thread using the specific drill size for the insert. This is NOT a standard tap drill. It is larger because the insert's external thread pitch and diameter differ from the original. Every insert kit specifies the exact drill diameter. Using the wrong drill produces an oversize or undersize hole that causes the insert to be loose or the tap to bind.

Step 2: Tap. Tap the hole with the STI (Screw Thread Insert) tap provided with the insert kit. The STI tap cuts the oversize internal thread that receives the insert's external coils. STI taps look similar to standard taps but cut a thread sized for the insert, not for the bolt. Using a standard tap instead of the STI tap is the most common installation mistake. The resulting thread is the wrong pitch diameter for the insert.

Step 3: Install the insert. Thread the insert onto the installation mandrel (a tool with a drive tang that engages the insert's driving notch). Wind the insert into the tapped hole by turning the mandrel clockwise. The insert should wind in smoothly with moderate resistance. If it requires excessive force, the hole is undersize or the tap is worn. The insert should sit 1/4 to 1/2 turn below the surface of the hole.

Step 4: Break the tang. Wire thread inserts have a drive tang at the bottom that the mandrel engages. After installation, the tang must be broken off using the tang break tool (a punch-like tool that fits into the hole and snaps the tang at its designed notch). The broken tang falls into the hole if it is a through hole, or must be retrieved with a magnet or air blast if it is a blind hole. Missing this step leaves a tab that interferes with bolt engagement.

Insert Material Selection

18-8 stainless steel (302/304): The standard material for wire thread inserts. Compatible with most parent materials including aluminum, steel, cast iron, and magnesium. Good corrosion resistance and temperature performance to about 800°F. This is the default choice for 90% of applications.

Phosphor bronze: Used when stainless steel would cause galvanic corrosion with the parent material (unlikely in most applications) or when non-magnetic properties are required. Also used in certain electrical applications where the insert must conduct electricity. Less common than stainless and more expensive.

Inconel 625 / X-750: High-temperature alloys for applications above 800°F. Used in exhaust systems, gas turbines, and high-temperature process equipment. Inconel inserts maintain thread strength at temperatures that would soften stainless steel. Expensive and require matching STI taps in the correct material grade.

Carbon steel: Occasionally used for solid inserts in steel parent materials where corrosion is not a concern. Carbon steel inserts can be heat-treated for higher strength. Not recommended for dissimilar-metal contact in corrosive environments due to galvanic corrosion risk.

For the parent material, the key consideration is whether the material can hold the oversize thread. Aluminum, cast iron, and most steels work well. Very soft materials (pure copper, lead alloys, some plastics) may not develop sufficient thread strength in the oversize hole to retain the insert. In those cases, use an adhesive (Loctite or similar) on the insert's external thread during installation to supplement mechanical thread retention.