Repair a thread with an insert
Plan a thread repair with wire or solid inserts. Check the kit drill, STI tap, insert length and hole depth before installation.
Select thread
FiltrosAll
What is damaged?
Repair details
Result · Recommended repair
Tap drill
Ø6.3 mm
Teórico Ø6.299 mm
Tap to use
M6 x 1 STI
Ø mayor alojamiento Ø7.299 mm
Minimum depth
≈ 10.5 mm
Inserto 1.5D · rosca útil 12 mm · agujero through
STI drill and tap worked out geometrically (nominal thread + 1.299×pitch, standard coil diametral offset): they reproduce the usual kit values, but the manufacturer's datasheet prevails.
⚠ Repair guidance: it does not replace the judgement of a qualified professional. Always check the drill, the STI tap, the depth and the result against the manufacturer's kit datasheet before machining.
Installation steps
- 1Taladrar a ≈Ø6.3 mm (mayor que el Ø nominal; el alojamiento STI aloja la bobina). Confirma con la ficha del kit.
- 2Roscar con el macho M6 x 1 STI del kit; nunca con el macho estándar de la misma medida.
- 3Enroscar la bobina con el mandril hasta 1/4-1/2 vuelta bajo la superficie y romper la lengüeta (tang) con punzón.
Recomendaciones
- 1D para carga ligera, 1.5D uso general, 2D o más en aluminio/carga alta.
- La bobina es más económica y rápida de instalar; el casquillo sólido resiste mejor cargas repetidas o vibración extrema.
- Agujero pasante: confirma que el inserto no sobresalga por el lado opuesto de la pieza.
El taladro y el macho los marca el kit del fabricante (HeliCoil, Recoil, Keensert…), no la tabla de roscado normal de M6 x 1. Los insertos reparan roscas interiores (agujeros roscados).
Workshop tips and common problems
Technique selection and troubleshooting for the method repair insert. Change the threading method above for specific tips.
How to do it right
Repair Inserts: Coil vs. solid bushing
HeliCoil/Recoil are wire coils: they need a specific STI tap and a tap drill larger than the nominal thread; the ordinary tap drill will NOT do. Keensert/Time-Sert are solid bushings: they usually resist pull-out and vibration better, but the tap and the hole depend on the manufacturer's datasheet and do not always match the STI standard.
Go/No-Go gauge inspection
For production valid with Go/No-Go gauge, flank micrometer (3 wires) or plane criterion. The correct drill does NOT guarantee an accepted thread - the tolerance is checked on the flank, not the diameter of the drill bit.
If something goes wrong · symptom → cause → solution
⚠ Repair insert does not grip or rotate crazy
Causes: the hole is too large, the wrong tap (you used an ordinary one instead of the STI or the kit's), or an oversized hole thread. Fix: drill and tap with the exact STI/specific tap and diameter from the kit datasheet, not those of the nominal thread; on solid bushings, apply the adhesive if the manufacturer specifies it.
⚠ The insert protrudes or sits too deep
Causes: insert length wrongly chosen for the thickness, or an incorrect hole depth. Fix: with a coil, fit it between 1/4 and 1/2 turn below the surface and snap off the tang; match the length (1D, 1.5D, 2D) to the thickness and the load, and on a through hole check it does not show on the far side.
⚠ The insert is torn off in soft material (aluminum)
Causes: an insert too short (1D) for the load, high tightening torque, or an already damaged base thread. Fix: use longer inserts (2D or more) or a solid bushing on joints that take high torque or repeated dismantling; this is a classic failure of the 1D HeliCoil in cylinder-head or crankcase threads.
Other threading tools
Technical guide: repair a damaged thread with insert (HeliCoil, Recoil, STI)
Reference article for this section. Switch goal above (drill, identify, CNC, method, repair or base plate) to see the guide for that tool.
When an internal thread strips, breaks or wears out from repeated assembly, repairing with an insert is almost always a better option than oversizing the original thread. The key is picking the right insert type and using the drill and tap the kit specifies, not those of the nominal thread.
The repair drill and STI tap prepare the insert hole, not the nominal thread. Identify the final size and confirm hole preparation and depth with the kit.
Coil (HeliCoil/Recoil) vs. solid bushing (Keensert/Time-Sert)
The wire coil (HeliCoil/Recoil) is the cheapest and quickest option to fit: it is screwed in with a mandrel to a quarter or half turn below the surface and the driving tang is snapped off. The solid bushing copes better with repeated loads, extreme vibration or frequent dismantling, but needs more machining and, on some models, structural adhesive.
Both types typically use an STI (Screw Thread Insert) thread profile on the outside of the insert, but the exact tap and drill always depend on the manufacturer's sheet for the specific kit, not a generic table.
Why the STI drill and tap are not those of the nominal thread
The drill diameter for an insert is larger than that of the original thread, because it must house the insert wall as well as the final internal thread. Using the standard nominal-thread drill (the one you would use with an ordinary tap) leaves the insert without enough grip in the base material.
Minimum depth: blind hole vs. intern
In a blind hole, the minimum drill depth must cover the insert length (usually given in diameters: 1D, 1.5D, 2D) plus a margin for the tap chamfer and chip evacuation. In a through hole that margin is smaller, but you must check the insert does not protrude on the far side of the part.
When NOT to use a metal insert
In brittle materials such as cast iron, drill and tap carefully: too much torque when fitting the insert can crack the material around the hole. In plastic parts, a metal HeliCoil-type repair insert is not the usual method: the norm is a heat-set insert (by heat or ultrasound), unless the kit says otherwise.
In aluminium and other soft materials — very common when repairing cylinder-head or crankcase threads — use longer inserts (2D or more), or consider a solid bushing if the joint will take high torque or repeated dismantling: a classic mistake is fitting a 1D HeliCoil in aluminium and watching it pull out on the first hard tightening.
Frequently asked questions: repair a damaged thread with insert (HeliCoil, Recoil, STI)
What do I need to repair a thread with HeliCoil or insert?
You need the drill specified by the insert kit (not the nominal thread's), an STI tap or the manufacturer's own, an insert of the right length for the load and material, and a gauge to verify the final result.
What drill do I need for an insert on an M8 thread?
It depends on the kit and the insert type (coil or solid bushing): it is always larger than the drill for an M8 with an ordinary tap. Check the datasheet of the specific kit's manufacturer; do not extrapolate from a standard tap table.
Does a HeliCoil insert work on aluminum?
Yes, and it is one of the most common uses, but in aluminium it is best to use a longer insert (2D or more) or consider a solid bushing if the joint takes high torque or is dismantled often, because aluminium is softer and the original thread is usually already damaged by overload.
What is the difference between a coil insert and a solid bushing?
The coil (HeliCoil/Recoil) is cheaper and faster to fit, with a tang that snaps off after installation. The solid bushing (Keensert/Time-Sert) resists pull-out and vibration better, but needs more machining of the hole and, on some models, adhesive.
Can I repair a spark plug thread or an electrical component with an insert?
Yes, there are dedicated kits for spark-plug threads and for special threads (electrical/conduit), but the seat, reach and compatibility with the actual component must be validated with the corresponding kit and gauge; extrapolating from a generic metric thread does not work.
Third-party trademarks and technical standards are cited for identification and descriptive purposes only; they belong to their respective owners and GetGCode is not affiliated with, authorised or endorsed by them. See the details in the Legal notice.
Related tools
Cutting parameters calculator
Cutting speed (Vc), rpm, feed and theoretical surface finish for turning, milling, drilling and threading by material and tool. A starting point.
GSM Code List
List of G and M Codes for CNC Programming. Search and filter ISO commands by control, machine or function.
CNC G-code Simulator
CNC IDE: editing with G/M tooltips, line inspector, conversion between controls and 2D/3D toolpath.