Definition
A self-locking nut that uses a fiber insert (typically nylon) seated in a collar at the top of the nut. As the nut is threaded onto a bolt, the bolt cuts threads into the unthreaded fiber, creating friction that resists loosening from vibration. Fiber stop nuts are limited to a maximum operating temperature (commonly around 250°F) and are generally restricted to non-rotating applications and locations where the bolt does not turn against the fiber.
Plain English
A nut with a soft plastic-like ring inside the top. When you screw it onto a bolt, the bolt squeezes into that ring tightly enough that the nut won't shake loose.
Context Anchor
Seen in aircraft maintenance when selecting, installing, inspecting, or replacing nuts on bolts in areas where vibration is a concern.
Derivation
"Stop" here means to stop the nut from backing off. The fiber insert is what does the stopping, hence "fiber stop nut."
Why Pilots Care
Maintains fastener security on structural and engine mounts, reducing risk of loosening in flight.
Intuition Check
Do not read “fiber stop nut” as a nut made entirely of fiber, or as a nut that permanently stops all movement. The fiber insert is the locking part; it adds grip so the nut resists loosening.
Example Sentence 1
The technician installed a new fiber stop nut on the control rod end, since reusing a self-locking nut is not permitted.
Example Sentence 2
Fiber stop nuts should be replaced after several removals because the fiber insert loses its locking ability.