Helicoil and Thread Insert Practices for Invar Threaded Holes

By Invar Machining editorial team· Published September 21, 2026 · How we write these guides

A tapped hole directly in Invar works fine for most applications, but repeated assembly and disassembly, or a hole in a thin section that needs more thread engagement strength than the base material reliably provides, is where a helical thread insert becomes worth the extra step. Knowing when to specify one, and how, avoids both unnecessary cost on holes that do not need it and stripped threads on ones that do.

Why thread inserts come up on Invar parts specifically

Drilling and tapping directly into Invar, covered in Invar drilling and tapping, works reliably when the hole is drilled and tapped correctly, but a few conditions push toward a thread insert instead of a direct tapped hole:

• Repeated assembly cycles. A hole that will be unscrewed and rescrewed many times over a part’s service life wears the threads with each cycle. An insert, typically a more wear-resistant material than the surrounding Invar, absorbs that wear and can be replaced if it eventually does wear out, rather than damaging the base part.
• Thin sections. A hole in a thin wall may not have enough material depth for a tapped thread to provide adequate engagement strength, discussed further in thin-wall Invar machining. An insert can provide better thread engagement in less material depth than a direct tap.
• Higher torque or load requirements than a direct tapped hole in Invar reliably handles, particularly relevant given Invar’s tendency to work harden around a drilled and tapped hole, covered in why Invar work hardens.

How helical thread inserts work

A helical thread insert, often referred to by the trade name Heli-Coil, is a coiled wire form installed into a slightly oversized, specially tapped hole. Once installed, it provides an internal thread matched to the fastener size while presenting a more standard, often more wear-resistant thread engagement than tapping directly into the base material. The installation hole is typically slightly larger than a standard tap drill for the same fastener size, since the insert itself takes up some of the thread depth.

Installation sequencing

Thread inserts are typically installed after the part has been fully machined, stress relieved and brought to final dimensions, not before or during the roughing sequence described in stress relief for Invar machined parts. Installing the insert into a hole that is already in its final, stable dimensional state avoids the risk of the surrounding material shifting slightly during a later stress relief cycle and affecting the insert’s fit or position.

Material considerations

Thread inserts are commonly available in various materials, and the choice affects both mechanical performance and, for some applications, galvanic compatibility with the surrounding Invar and the fastener being used. For most general applications this is a minor consideration, but for parts destined for humid, marine or otherwise corrosion-prone environments, discussed in surface finish and coating options for machined Invar, it is worth confirming the insert material is compatible with both the base metal and the fastener to avoid galvanic corrosion at the joint.

When to skip the insert

For holes that will be assembled once or only a handful of times over the part’s service life, and that carry moderate load, a direct tapped hole in Invar, done correctly with sharp tooling and a cutting tap, is usually sufficient and avoids the added cost and installation step of a thread insert. Reserving inserts for the holes that genuinely need them, rather than specifying them across the whole part by default, keeps cost proportionate to actual risk.

Specifying inserts on your drawing

• Call out which specific holes need thread inserts, rather than a blanket note for the whole part
• Specify the insert material if your application has a specific compatibility requirement
• Note the expected number of assembly cycles for holes where you are unsure whether an insert is warranted, and let the shop advise

Getting a quote

If your part has holes that will see frequent assembly and disassembly, or thin sections where thread engagement strength is a concern, mention it on the RFQ so the shop can recommend where inserts are worth the added step.

Send your drawing through our RFQ form, and we match you with shops that can advise on thread strategy for your specific holes, not just tap everything the same way by default.

Frequently asked questions

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