Repairing a Stripped Threaded Hole with Wire or Solid Inserts: Insert Choice, STI Tap Sizing and What Strength You Can Claim

Rule out re-tapping at nominal size before you touch the part

A wire insert does not go into an ordinary tapped hole of the same nominal size. The receiving thread has to be larger than the final internal thread, because the insert's own internal thread is what ends up at nominal ¹⁵. That means the hole is drilled to an insert-specific diameter and cut with a special STI (Screw Thread Insert) tap ¹⁵. Take an M6×1.0 final thread. The hole is prepared with an M6×1.0 STI tap, not a standard M6×1.0 tap ¹⁵. Running a normal metric tap instead makes the installation fail, and the insert will not seat correctly ¹⁵. Order the STI tap and the matching drill before you open up the damaged hole. Once you have cut a nominal-size thread there is nothing left for the insert to grip.

Wire insert or solid bushing: let the surrounding material decide

The wire (helical) insert is the commonly cited repair type for a stripped tapped hole because it requires the least hole enlargement ¹. A solid bushing insert is preferred if space permits ¹. Think of it as a material budget: the bushing takes a bigger bite out of the boss, so it is only on the table where the part can give that material up. Inserts in general are used to give a strong, wear-resistant tapped hole in soft material and to repair stripped threads ¹⁰. The threaded families are the wire thread, the self-tapping and the solid bushing ¹⁰. For a repair on a part you cannot easily replace, try the least-enlargement option first. If the boss is generous and you want a solid threaded body, the bushing is the alternative the sources name.

Size the insert from its external thread, not from the bolt

The insert's external thread is nominally 1/8 in. larger in diameter than the internal thread it presents ¹⁰. The classic illustration is a 10-32 bolt carried in an insert whose external thread matches an equivalent 5/16-18 nut ¹⁰. So you pick the tap from the insert, and the insert from the bolt — never the tap from the bolt. That relationship is the whole sizing rule the sources give. They stop short of a drill-size table, a minimum boss diameter, an edge distance or a wall-thickness rule for repaired holes. Get the drill diameter and the minimum surrounding material from the insert supplier's catalogue for the exact insert part number. Check the boss on your part against it before drilling.

Installation sequence and tooling checks

The sequence is fixed by the insert design: drill, STI tap, install the insert, then break the tang off at its notch ¹¹⁵. The tang is part of the insert, not of the finished thread, so the repair is not complete until it is gone.

• Confirm the STI tap and drill match the insert part number, not the bolt thread ¹⁵
• Check the hole is drilled and tapped oversize before the insert goes in ¹⁵
• Install the insert, then break the tang off at the notch ¹
• Ask the supplier for the installation tooling and seating method for that insert type — the sources describe only the STI tap and the tang break-off

What you can honestly claim about the repaired thread

An insert can restore a damaged or stripped threaded connection and can add thread retention, but the complete fastening system still has to be designed for its service conditions ¹⁶. The insert is a component in the joint, not a substitute for joint design. What the sources will not give you is a number. There is no quantified pull-out, tensile or fatigue recovery for a repaired hole against the original thread, and no test basis behind one. There is no insert standard named, no qualification requirement, and no post-repair acceptance method such as a go/no-go gauge or a torque test. If your procedure needs a strength figure or a pass/fail criterion, get it from the insert supplier's qualification data and your own inspection plan — not from a rule of thumb.

When a self-tapping solid insert is admissible

Self-tapping inserts are mostly solid bushings with a tapered external thread like a self-tapping screw ¹. The thread-forming versions generate no cutting chips because they form the thread rather than cut it, but they can only be used in softer materials ¹. That chip-free forming action is the reason to consider one, and the same forming action is the limit on where it works. If your parent material is hard, the self-tapping route is not the one the sources support. If it is soft and you want to avoid chips, ask the supplier whether their self-tapping insert is rated for your specific parent material.

FAQ

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