FAA Aviation Mechanic General (AMG)Materials, Hardware and ProcessesHard
A mechanic is comparing thread tolerances between an AN aircraft bolt and a commercial-grade bolt of the same size. Aircraft bolts such as AN and MS bolts are typically manufactured to which thread class fit?
- AClass 1 (loose fit)
- BClass 2 (free fit)
- CClass 4 (interference fit)
- DClass 3 (close fit)
Show answer & explanationAnswer & explanation
Correct answer: D. Class 3 (close fit)
AN and MS aircraft bolts are manufactured to a Class 3 thread fit, which provides closer tolerances than the Class 2 fit typically used on commercial hardware. This tighter tolerance improves fastener strength and reliability in critical aircraft applications.
Why the other options are wrong
- A. Class 1 fits are loose fits not used for structural aircraft hardware.
- B. Class 2 is a standard commercial free-fit tolerance, looser than aircraft-grade fasteners.
- C. Class 4 interference fits are used for special press-fit applications, not general bolts.
AN/MS Bolt Thread Class
AN and MS aircraft bolts use a Class 3 (close-tolerance) thread fit for greater precision and strength compared to commercial Class 2 fasteners.
- Class 3 = close fit, tighter tolerances than commercial hardware
- Commercial bolts commonly use Class 2 (free fit)
- Tighter thread fits reduce play and increase fatigue resistance
Memory trick: 'Aircraft bolts get the tightest hug' - Class 3 close fit for precision