FAA Aviation Mechanic General (AMG)Materials, Hardware and ProcessesHard
A technician is selecting a self-locking nut for installation on an engine exhaust clamp that will be exposed to temperatures exceeding 400°F. Which type of self-locking nut should be selected?
- AWing nut with a spring clip
- BAll-metal (elastic stop) lock nut
- CPlain hex nut with a lock washer
- DFiber (nylon) insert lock nut
Show answer & explanationAnswer & explanation
Correct answer: B. All-metal (elastic stop) lock nut
Fiber (nylon) insert lock nuts are limited to service temperatures below about 250°F because the nylon insert degrades at higher temperatures, losing its locking action. All-metal self-locking nuts use a deformed metal thread section to create locking friction and can withstand much higher temperatures, making them suitable for high-heat areas like exhaust systems.
Why the other options are wrong
- A. Wing nuts with spring clips are used for quick-access panels, not high-temperature structural applications.
- C. A plain hex nut with a lock washer is not a self-locking nut and is not standard practice for critical aircraft fasteners.
- D. Nylon inserts break down and lose locking ability above roughly 250°F, unsuitable for exhaust-area temperatures.
Self-Locking Nut Temperature Limits
Fiber-insert self-locking nuts are limited to lower temperatures (below about 250°F) because the nylon insert degrades, while all-metal self-locking nuts can be used in high-temperature areas such as exhaust systems.
- Fiber insert locknuts: limited to ~250°F max
- All-metal locknuts: suitable for high-temperature areas (engine, exhaust)
- Locking action in all-metal nuts comes from distorted threads, not an insert
Memory trick: 'Nylon melts, metal survives the heat' - choose all-metal near hot exhausts