Why Thread Holes Strip

Thread stripping happens when the internal thread yields or tears under load. In soft materials (aluminum, magnesium, zinc die castings, engineering plastics), the thread depth is shallow and the material is weak, so even a moderate over-torque can strip the first few threads. Repeated assembly and disassembly also wears the soft thread until it no longer holds. In harder materials, stripping usually means the bolt was stronger than the nut, or the engagement length was too short.

The economic stakes are high: an expensive die-cast housing or machined part with a stripped thread hole is not always worth scrapping. Repairing the thread in place is often cheaper and faster than replacing the part. The choice of repair method depends on the material, the load, whether the original bolt size must be preserved, and whether the repair must survive repeated re-assembly.

Option 1: Drill Out and Tap One Size Larger

The simplest repair is to drill the stripped hole oversize and tap it to the next standard thread size. For example, a stripped M6 hole can be drilled out to M7 or M8, tapped, and the original M6 bolt replaced with an M7 or M8 bolt. This is cheap, requires no special parts, and is strong if the wall thickness allows.

The downsides: the bolt size changes, so the mating part must also accommodate the larger bolt; you lose the original fastener specification; and the oversize hole may break through thin walls. This method is best for one-off repairs where changing the bolt size is acceptable and wall thickness is adequate. It is not suitable for production parts where the bolt size must remain standard.

Option 2: Wire Thread Insert Repair

The wire thread insert (Helicoil-style) repair is the production-grade solution, covered by GB/T 24425.1-2009 (ordinary wire thread inserts), which applies to strengthening tapped holes in aluminum, magnesium, copper alloy, cast iron and non-metal bodies and to repairing damaged threads. The stripped hole is drilled out to the STI oversize diameter, tapped with a special STI tap, and a stainless steel wire thread insert is installed. The insert forms a standard internal thread at the original nominal size, so the original bolt and bolt specification are preserved. The stainless steel insert also strengthens the thread against future stripping.

This is the preferred repair for aluminum and magnesium die castings where the original bolt size must be maintained. It requires STI taps and insert installation tools, but the parts are cheap and the process is well-established. It is also the method of choice for repair during production, where a stripped thread would otherwise scrap an expensive housing.

Option 3: Slotted Insert and Welded Plug

A slotted insert (also called a keensert or screwed insert) is a solid threaded bushing with external threads and slots. It is screwed into a tapped hole and the slots are staked to lock it. It provides a more robust repair than a wire thread insert in very soft or heavily loaded applications. A welded plug repair involves welding over the stripped hole, re-drilling, and re-tapping - this is a last resort because welding can distort the part and changes the material properties locally.

How to Choose Among Four Repair Methods

How to Prevent Thread Stripping

Prevention is better than repair. For soft materials, specify longer engagement length (at least 1.5d in aluminum, 2d in magnesium). Consider a wire thread insert from the design stage rather than retrofitting after stripping. Control assembly torque to the correct value, selected per GB/T 16823.2 (general rules for tightening threaded fasteners); most stripping is caused by over-torque. Use a torque wrench or torque driver on soft-material joints. For plastic housings, use threaded inserts molded in or ultrasonically installed rather than tapping plastic directly. For repeated assembly, design for a prevailing-torque or insert solution that survives many cycles.

When to repair versus when to scrap. The decision to repair a stripped thread depends on the cost of the part and the cost of repair. For a cheap housing, scrapping is simpler. For an expensive die-cast or machined part, repair is almost always worth it. Wire thread insert repair is fast, cheap, and reliable. The key is to catch stripping early: if a threaded hole starts to feel soft or the bolt turns freely at low torque, stop and repair before the thread is completely destroyed. A partially stripped hole is much easier to repair than one where the thread is gone and the hole is enlarged.

Prevention in the design stage. The cheapest thread repair is the one you never need. Design for adequate engagement: in aluminum, at least 1.5 times the bolt diameter; in magnesium, at least 2 times; in plastic, use molded-in inserts. Control assembly torque with a torque-limiting driver to prevent over-torque stripping. For joints that are assembled and disassembled repeatedly, specify a wire thread insert from the start rather than repairing after stripping. For soft die castings, consider whether the thread holes should be drilled and tapped after casting rather than as-cast. These design choices prevent stripping before it starts, which is always cheaper than fixing it after.

Repair vs replace economics. A simple economic calculation: if the part costs $500 to replace and a wire thread insert repair costs $50 in parts and labor, repair is obvious. If the part costs $5 to replace, scrap it. The break-even depends on part cost, repair time, and whether the repair can be done in place. For assembled equipment where disassembly is expensive, even a cheap part may be worth repairing in place. Keep a repair kit of common wire thread inserts and STI taps on site so that a stripped thread does not stop a production line. The few hundred dollars of kit inventory saves thousands in downtime.

When to repair versus when to scrap. The decision to repair a stripped thread depends on the cost of the part and the cost of repair. For a cheap housing, scrapping is simpler. For an expensive die-cast or machined part, repair is almost always worth it. Wire thread insert repair is fast, cheap, and reliable. The key is to catch stripping early: if a threaded hole starts to feel soft or the bolt turns freely at low torque, stop and repair before the thread is completely destroyed. A partially stripped hole is much easier to repair than one where the thread is gone and the hole is enlarged.

Prevention in the design stage. The cheapest thread repair is the one you never need. Design for adequate engagement: in aluminum, at least 1.5 times the bolt diameter; in magnesium, at least 2 times; in plastic, use molded-in inserts. Control assembly torque with a torque-limiting driver to prevent over-torque stripping. For joints that are assembled and disassembled repeatedly, specify a wire thread insert from the start rather than repairing after stripping. For soft die castings, consider whether the thread holes should be drilled and tapped after casting rather than as-cast. These design choices prevent stripping before it starts, which is always cheaper than fixing it after.

Repair vs replace economics. A simple economic calculation: if the part costs $500 to replace and a wire thread insert repair costs $50 in parts and labor, repair is obvious. If the part costs $5 to replace, scrap it. The break-even depends on part cost, repair time, and whether the repair can be done in place. For assembled equipment where disassembly is expensive, even a cheap part may be worth repairing in place. Keep a repair kit of common wire thread inserts and STI taps on site so that a stripped thread does not stop a production line. The few hundred dollars of kit inventory saves thousands in downtime.