Helicoil Inserts

What are Helicoil inserts? Helicoil inserts are thread repair inserts made from diamond section stainless wire wound into a coil. Installed into a hole tapped with a matching STI tap, the coil springs outward against the tapped thread while presenting a precise standard thread on the inside, so the hole returns to its original size. Because a wire coil adds very little wall thickness, it works where there is not much material to give up. Helicoil is a registered trademark that has become the generic term for the style. AlbanyCountyFasteners.com stocks these in 18-8 and 303 stainless, from #2-56 through 1-1/8.

Helicoil Inserts

How does a Helicoil insert work?

The sequence matters and it trips people up. You drill the damaged hole oversize, then tap it with an STI tap, which is not a standard tap in the nominal size but one cut specifically to receive the insert. The coil is wound slightly larger than the tapped hole, so as the installation tool drives it in by the tang at its base, it compresses and springs back against the tapped thread, locking in by friction across the full engagement length. The inside of the coil then presents a standard thread in the original size. Last step, you snap off the tang with a punch, since leaving it in place blocks the bolt from seating fully.

Why choose Helicoil wire inserts

Restores the hole to its original thread size, so the original fastener still fits
Adds minimal wall thickness, suiting thin bosses and tight spaces
Hardened stainless thread surface resists wear, galling, and seizing
Spreads load across the full engagement length rather than concentrating it
Available for virtually every standard inch and metric thread size
The most economical repair for the majority of stripped thread jobs

Materials and sizes stocked

18-8 stainless in unified inch sizes, coarse and fine, from #2-56 through 1-1/8
303 stainless in metric sizes including M6 and M14
Lengths specified as a multiple of diameter, commonly 1.5D and 2D
Choose 2D length for soft parent material or higher load, 1.5D for general repair

Common applications

Repairing stripped threads in aluminum engine blocks, heads, and casings
Reinforcing threads in soft metals before they fail
Automotive, motorcycle, and small engine repair
Machine tool and equipment maintenance
Aerospace and precision assemblies where thread integrity is critical
Converting a damaged hole between inch and metric threads

You will need an STI tap and an installation tool sized to the insert, available in our thread insert installation tools. Where the repair needs more pull-out strength than a wire coil provides, consider solid thread inserts for metal.

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Helicoil Insert FAQs

What is an STI tap and why do I need one?

STI stands for Screw Thread Insert. It cuts a thread sized to receive the coil, which is larger than the nominal thread the insert will ultimately provide. A standard tap in the nominal size will not work, and this is the most common mistake people make on their first insert repair.

What does 1.5D and 2D mean?

Insert length expressed as a multiple of the thread diameter. A 1.5D insert in a 1/4 inch thread is about 0.375 inch long. Use 1.5D for general repair in steel or cast iron, and 2D where the parent material is soft, such as aluminum, or where the joint carries higher load.

Do I have to break off the tang?

Yes. The tang is the small drive lug at the bottom of the coil that the installation tool turns against. Left in place it blocks the bolt from passing fully through, so snap it off with a punch after installation and make sure the broken piece is removed from the hole.

Are Helicoil repairs as strong as the original thread?

Often stronger, particularly in soft material. A hardened stainless coil in an aluminum boss gives a better thread surface than the aluminum did, and spreads load more evenly across the engagement. The limit is the parent material around the insert rather than the insert itself.

Can I install a Helicoil without a kit?

You need the correct STI tap and an installation tool at minimum, plus a drill in the specified oversize. A kit bundles these matched to one thread size, which is usually the practical way to buy for a single repair.

18-8 or 303 stainless?

Both are austenitic stainless with similar corrosion resistance. 303 is a free-machining grade with sulfur added for cleaner cutting, which is why it appears in machined and metric insert lines. For thread repair purposes the practical difference is minimal.