Lockwire / Safety Wire
How to Safety Wire Fasteners
The correct technique prevents fastener loosening in high-vibration assemblies — used in aerospace, motorsport, marine, and industrial maintenance. Wire direction, twist count, and pigtail length all matter.
Key Rule
Wire must always be tensioned such that any loosening of the fastener would tighten the wire, not loosen it. Direction of wrap matters — an incorrectly routed wire that loosens when the fastener tries to back out provides no safety function whatsoever.
What You Need
- Safety wire — MS20995C32 (0.032") is the most common for general fasteners
- Safety wire pliers (twisting tool)
- Wire cutters / diagonal cutting pliers (dykes)
- Clean fasteners with drilled heads, or castellated nuts with cotter pin holes
Double-Twist Method (Most Common)
The double-twist method is required by most aviation and industrial maintenance standards. It produces a consistent, auditable result that inspectors can verify at a glance.
- 1
Select wire diameter
Use 0.032" wire (MS20995C32) for most AN/NAS bolts and standard fasteners. Use 0.020" for small fasteners under M6 or where the bolt head hole is too small. Check the engineering drawing or maintenance manual for the specified diameter.
- 2
Cut wire to length
Allow approximately 12 inches of wire per fastener pair — err long, you can always cut the excess. For groups of three or more fasteners, add roughly 6 inches per additional fastener.
- 3
Thread wire through the first fastener
Pass the wire through the drilled hole in the fastener head, or thread through the groove of a castellated nut. Pull through so both wire legs are roughly equal in length.
- 4
Cross and loop — establish tension direction
Bring both wire ends together and fold one over the other so that tension opposes loosening. For a standard right-hand bolt, the wire must exit the hole going clockwise when viewed from the bolt head — any loosening rotation must pull the wire tighter, not looser.
- 5
Twist the pigtail with safety wire pliers
Use safety wire pliers to twist both legs together at 6–8 twists per inch. Keep tension consistent. Do not over-twist — the wire will work-harden and snap. A smooth, uniform pigtail indicates correct technique.
- 6
Route to the next fastener
Keep the wire taut as you route to the next fastener. No slack loops allowed between fasteners. Maximum span between safety-wired fasteners is generally 6 inches per FAA AC 43.13.
- 7
Finish the final pigtail
After threading the last fastener, cut wire leaving 3–4 twists past the hole (about 3/8"). Bend the tail flat against the assembly so it cannot snag gloves, hoses, or adjacent components.
Single-Wrap Method (Castellated Nuts)
When a castellated nut is used and there is insufficient clearance for a double twist, the single-wrap method is acceptable. Thread the wire through the nut groove and the corresponding bolt hole in a single pass, then bend the tail down against the assembly. This method is common on fuel fittings and close-quarters hardware where the double-twist geometry will not fit. Confirm that the applicable maintenance manual permits single-wrap before using it.
Common Mistakes
Wrong direction
Wire that loosens when the fastener tries to unscrew provides zero safety function. Always verify that loosening rotation of the bolt increases wire tension before moving to the next fastener.
Too few twists
Fewer than 6 twists per inch allows slack to develop under vibration. The wire may still look installed but will not prevent loosening. Count twists per inch — do not estimate.
Pigtail too long
Long pigtails snag on hoses, wiring bundles, and moving parts. They can also puncture gloves or skin during subsequent maintenance. Cut to 3–4 twists and bend the tail flat.
Reusing wire
Never reuse safety wire. Twisting work-hardens the metal and causes micro-fatigue that is invisible to the eye. Reused wire will snap under vibration. Always start with fresh wire.
Choosing the Right Alloy
MS20995C32 stainless steel is the correct choice for most aerospace and general industrial fastener applications. For saltwater, offshore, or chloride-rich environments, specify Monel (NC suffix — e.g., MS20995NC32) which resists the stress-corrosion cracking that degrades stainless over time. For temperatures above 1,200°F — turbine hot sections, afterburner hardware, exhaust manifold bolts — Inconel (N suffix) is required.
Full alloy and diameter reference →Frequently Asked Questions
Which direction should safety wire go?+
Wire must tighten when the fastener tries to loosen. For standard right-hand (clockwise-tightening) bolts, the wire must exit the drilled hole going in the clockwise direction when viewed from the bolt head. Any loosening rotation of the bolt must increase wire tension.
How many twists per inch for safety wire?+
6 to 8 twists per inch is the standard for double-twist safety wire. Tighter is better, but do not over-twist — work-hardened wire loses ductility and will snap under vibration. A smooth, uniform pigtail at 6–8 twists per inch is the goal.
What diameter safety wire should I use?+
0.032" (MS20995C32) is correct for most AN/NAS bolts and general fastener applications. 0.020" is used for small fasteners below M6 or where the bolt head hole will not accept 0.032". Always check the engineering drawing or maintenance manual — the specified diameter is not optional.
Can safety wire be reused?+
No. Once twisted, safety wire has been work-hardened and fatigued. The metal fatigue is not visible to the eye but is real — the wire will be brittle and prone to snapping under vibration. Always install fresh wire on every job.
What is the maximum span between safety-wired fasteners?+
Generally 6 inches per FAA Advisory Circular AC 43.13-1B. This is a widely adopted industry standard, but always verify against your specific maintenance manual or OEM documentation for the equipment you are working on.
Need Safety Wire for Your Application?
American Wire Works stocks MS20995 lockwire in all seven alloys and five diameters, available in 1 lb cans and larger spools. Browse safety wire →