18-8 Stainless Steel Helicoil Inserts

What are 18-8 stainless Helicoil inserts? These are wire thread repair inserts coiled from diamond section 18-8 stainless, covering unified inch threads in both coarse and fine series. 18-8 is the workhorse austenitic stainless: corrosion resistant, tough, and hard enough to give a thread surface that outlasts the aluminum or mild steel it is installed into. Installed with an STI tap and driver, the coil restores a stripped hole to its original thread size. AlbanyCountyFasteners.com stocks these from #2-56 through 1-1/8 in 1.5D and 2D lengths.

18-8 Stainless Steel

Internal Thread:

Why 18-8 stainless for thread repair?

Because the insert has to outlast the hole it is fixing. Most thread repairs happen in aluminum, and the reason the thread failed is that aluminum is soft, galls against steel fasteners, and wears with every assembly cycle. Dropping a stainless coil into that hole changes the contact surface entirely: the bolt now threads against hardened stainless instead of soft aluminum, so the repair does not simply fail again in the same way. 18-8 adds corrosion resistance, which matters because a repaired hole often sits in an engine casing, an outdoor machine, or a marine assembly where a plated steel insert would rust and seize the bolt in place.

Why choose 18-8 stainless Helicoil inserts

Hardened thread surface far more wear resistant than aluminum or mild steel
Corrosion resistance for engine, outdoor, and marine repairs
Resists galling and seizing that soft parent material suffers from
Spreads load across the full engagement length
Widest inch size range available, coarse and fine

Sizes stocked

Machine screw sizes including #2-56, #3-48, #4-40, #5-40, #6-32, #8-32, #10-24, #10-32, #12-24, and #12-20
Fractional coarse including 1/4-20, 5/16-18, 3/8-16, 7/16-14, 1/2-13, 9/16-12, 5/8-11, 3/4-10, and 7/8-9
Fractional fine including 1/4-28, 5/16-24, 3/8-24, 7/16-20, 1/2-20, 9/16-18, 5/8-18, 3/4-16, and 7/8-14
Large sizes including 1-8, 1-12, 1-14, and 1-1/8
Available in 1.5D and 2D lengths, so match length to parent material and load

Common applications

Aluminum engine blocks, cylinder heads, and transmission casings
Spark plug holes and other high heat threaded connections
Motorcycle, marine engine, and small engine repair
Machine tool spindles, fixtures, and equipment maintenance
Reinforcing threads in new soft metal assemblies before they strip
Any repair where the alternative is scrapping the part

For metric threads, see our metric 303 stainless Helicoil inserts. You will need a matching STI tap and driver from our installation tools.

Related Categories

18-8 Stainless Helicoil FAQs

What size drill and tap do I need?

Both are specified per insert size and both are oversize relative to the nominal thread. Drill to the size listed for the insert, then tap with the matching STI tap. Do not use the standard tap drill chart for the nominal thread, since that hole will be far too small.

Should I use 1.5D or 2D?

1.5D covers general repair in steel and cast iron. Choose 2D when the parent material is aluminum or another soft metal, or when the joint sees higher load, since the extra engagement length spreads stress over more of the boss.

Can I use these in a spark plug hole?

Yes, wire inserts are commonly used for spark plug thread repair, and stainless handles the heat well. For high value engines or where the repair must be certain, a solid insert designed for spark plug holes gives more thread contact area.

Will the bolt still torque to spec?

Generally yes, and often the repaired thread holds better than the original in soft material. Confirm the parent boss is sound around the insert, since the limit is the material the insert is anchored in rather than the insert itself.

How do I know if the insert is seated correctly?

The top coil should sit one quarter to one half turn below the surface of the hole. Sitting proud means it will interfere with the mating part, and sitting too deep reduces thread engagement for the bolt.

Are these the same as Heli-Coil brand?

These are wire thread inserts of the same type and standard sizing. Heli-Coil is a registered trademark of Newfrey LLC, and the term is widely used generically for this insert style across the industry.