The Problem with Threads in Soft Metals

Aluminum, magnesium, and even some grades of stainless steel share a common weakness: their threads are fragile. Repeatedly driving a steel bolt into an aluminum housing strips threads. Stainless fasteners seize together. Soft metals gall, deform, and fail in ways that steel-on-steel connections don’t.

Thread inserts and proper fastening practices solve these problems. Here’s what you need to know.

What Is Galling?

Galling is a form of cold welding. When two metal surfaces slide against each other under pressure — like threads being tightened — microscopic high points fuse together, tear free, and re-fuse further along. The result is a seized fastener that won’t tighten or loosen without destroying the threads.

Galling is most common with:

  • Stainless steel on stainless steel — The #1 offender. Austenitic grades (304, 316) are notorious.
  • Aluminum on steel — The softer aluminum deforms and smears against the harder steel threads.
  • Titanium on titanium — Similar mechanism to stainless, worse in some alloys.

What Causes Galling?

  • Speed — Fast installation with impact drivers dramatically increases galling risk
  • Pressure — Over-torquing pushes thread surfaces together harder
  • Surface finish — Rough or damaged threads create more friction points
  • Identical materials — Same-on-same galls more than dissimilar pairs
  • Lack of lubrication — Dry stainless threads are a galling guarantee

Prevention: Anti-Seize Compounds

Anti-seize is the first line of defense against galling. A thin film of lubricant (usually containing copper, nickel, or molybdenum disulfide particles) prevents metal-to-metal contact between thread surfaces.

When to Use Anti-Seize

  • Any stainless-on-stainless connection
  • Stainless bolts into aluminum housings
  • Any fastener that will be removed and reinstalled
  • High-temperature applications where lubricants would burn off
  • Dissimilar metal joints where galvanic corrosion is a concern

The Torque Problem with Anti-Seize

This is critical and widely misunderstood. Published bolt torque specifications assume dry threads. When you lubricate a fastener with anti-seize, you reduce friction — which means the same torque produces significantly more clamping force (preload).

If you apply the published dry torque to a lubricated bolt, you will over-stress the fastener. The bolt stretches more than intended, and you risk:

  • Yielding or breaking the bolt
  • Stripping threads in the mating part
  • Crushing gaskets or soft components

Rule of thumb: Reduce torque by 20–30% when using anti-seize or any thread lubricant. Some sources recommend specific K-factors:

  • Dry steel: K ≈ 0.20
  • Anti-seize (copper-based): K ≈ 0.13–0.15
  • Oil/grease: K ≈ 0.15–0.18
  • Waxed (e.g., cadmium-plated): K ≈ 0.12–0.14

The torque-tension relationship: T = K × D × F (where T = torque, D = nominal bolt diameter, F = desired preload). A lower K means the same torque produces more preload — so you must reduce T to hit the same F.

Thread Inserts: Helicoils and Alternatives

What They Do

Thread inserts provide a hard, wear-resistant thread surface inside a softer parent material. They accomplish three things:

  1. Reinforce weak threads — Aluminum tapped holes become as strong as steel
  2. Repair damaged threads — Strip a hole? Drill oversize, install an insert, and you’re back in spec
  3. Prevent galling — The insert material (typically stainless or phosphor bronze) eliminates soft-on-soft contact

Helicoil (Wire Thread Inserts)

The most common type. A Helicoil is a coiled wire made from diamond-shaped stainless steel wire that screws into a specially tapped hole (STI tap, slightly larger than standard). Once installed, it provides a standard internal thread.

Advantages:

  • Thin wall — minimal material removal from the parent part
  • Excellent load distribution across all thread turns
  • Available in free-running (standard) and screw-locking (with a grip tang) versions
  • Standard sizes from #2-56 through 1-1/2″–6
  • Temperature resistant to 800°F+ (stainless)

Installation:

  1. Drill to the specified oversize diameter
  2. Tap with an STI (Screw Thread Insert) tap — NOT a standard tap
  3. Wind the insert in with the installation tool until it sits 1/4 to 1/2 turn below the surface
  4. Break off the installation tang (if present) with a tang break tool or punch

Keenserts / Key-Locking Inserts

Solid inserts with external keys that lock into slots in the parent material. Stronger than Helicoils for heavy vibration and high-load applications. The keys deform into the parent material, preventing rotation and pull-out.

Best for: Structural joints, engine blocks, heavy equipment, anything with high cyclic loading

E-Z Lok and Press-In Inserts

Solid brass or stainless inserts that thread or press into the parent hole. Simpler than Helicoils — no special tap needed for press-in types. Common in plastics, wood, and soft metals where thread engagement is minimal.

Time-Sert

A solid bushing-style insert that provides more thread contact area than a Helicoil. Popular for spark plug hole repair and other critical applications where a wire insert isn’t robust enough.

When to Specify Thread Inserts

  • Aluminum housings with steel bolts — Especially if the joint will be serviced (opened/closed repeatedly)
  • Any soft material with threaded holes — Magnesium, zinc die-cast, plastics
  • High-vibration assemblies — Locking inserts prevent loosening without thread-locking adhesives
  • Thread repair — Stripped holes in expensive castings are cheaper to insert than to scrap
  • Where galling is a design concern — Stainless inserts in aluminum eliminate the issue entirely

Design Recommendations

For New Designs

  • Specify Helicoils or keyed inserts for all threaded holes in aluminum that will see steel fasteners
  • Call out the insert on the drawing — part number, depth, and installation requirements
  • Design the boss diameter large enough to accommodate the oversize drill (typically 10–15% larger than standard tap drill)

For Galling Prevention

  • Use anti-seize on ALL stainless-on-stainless connections — no exceptions
  • Reduce torque 25% from dry spec when using anti-seize
  • Slow down installation — hand-start all stainless fasteners, avoid impact drivers
  • Consider dissimilar materials (e.g., bronze nuts on stainless studs)
  • Use silver-plated or waxed stainless fasteners for repeated-use applications

For Thread Repair

  • Helicoils are the standard for most repairs — cheap, fast, and the repaired thread is often stronger than the original
  • Keenserts for structural or high-load repairs where you can’t afford any insert movement
  • Time-Serts for precision applications (spark plugs, sensors) where concentricity matters

Bottom Line

Threads in soft metals need protection. Anti-seize prevents galling in service. Thread inserts prevent failure by design. Both are cheap insurance against expensive problems — stripped castings, seized bolts, and field repairs that cost 100× what the insert would have cost upfront.