"The Bolt Just Spins" - Reinforce Aluminum Threads with Helicoil Inserts

Aluminum thread stripping causes, Helicoil and Key-Sert installation, correct tap hole sizing

Bolts in aluminum assemblies eventually start spinning freely — stripped threads. Aluminum shear strength is roughly 1/3 of steel, so even deep taps have limits under repeated use. The solution: thread inserts.

1. Why Aluminum Threads Strip Easily

Material strength gap: A6061 tensile ≈ 270 MPa vs. SS400 ≈ 400 MPa — shear strength difference is even larger.
Repeated assembly: Each cycle accumulates plastic deformation in thread flanks.
Over-torquing: Applying steel torque specs to aluminum causes instant stripping. Use ~60–70% of steel recommended torque for aluminum.

2. Helicoil Installation Procedure

Helicoils are coiled stainless-steel wire inserts.

Process: ① Drill oversized hole → ② Tap with special tap → ③ Install insert → ④ Break off tang
Hole sizing: Use dedicated oversize drill/tap (e.g., for M6 bolt: Ø6.6 drill → special Ø7 tap → M6 Helicoil)
Strength result: Post-installation tensile strength equals or exceeds base aluminum; thread shear strength actually exceeds steel

3. Helicoil vs. Key-Sert (Solid Insert)

Helicoil: General purpose, cost-effective, works in thin walls, sits flush after installation
Key-Sert / E-Sert (solid insert): Higher strength requirement, anti-rotation keys prevent pullout, better for high-vibration environments

4. Prevention by Design

When tapping aluminum, design thread depth at 2.5–3× bolt diameter from the start
For frequently disassembled joints (covers, inspection panels), plan for Helicoil inserts in original design

💡 Expert Advice
"Keep a Helicoil kit on hand. When a stripped aluminum part needs weeks of lead time for replacement, a Helicoil repair takes 30 minutes on the floor. Essential stock for any shop working heavily with aluminum."

Helical Inserts in US Manufacturing

Helical wire inserts (Heli-Coil® brand from STANLEY Engineered Fastening) are the dominant aluminum thread reinforcement in US aerospace, automotive, and industrial machinery. MS122xx and NAS1130 series cover military/aerospace use. For commercial production, NSI and Recoil® offer drop-in replacements at lower cost. ASME B18.29.1 covers non-aerospace dimensional standards.

Pre-Assembled vs Bare Insert

Tangless inserts (no break-off tang) are increasingly preferred for aluminum housings — eliminates the recovery step after installation. Time-Sert® keylock inserts (Time Fastener Co.) provide stronger pull-out resistance for high-torque service but cost ~3× a Heli-Coil. For repair work on stripped aluminum threads, Time-Sert is the US shop-floor standard.