Aircraft bolts are usually manufactured with a

• A. Class 1 fit for the threads
• B. Class 3 fit for the threads
• C. Class 2 fit for the threads
• D. Class 4 fit for the threads

Answer: (B)

Thread fit class controls how tightly bolt threads mesh with their mate, shaping how much clearance or interference there is. In aircraft joints, you want a fit that provides reliable load transfer and resistance to vibration, but still allows for reasonable assembly.

Class 3 is a close fit, offering less clearance than looser classes but not so tight as to cause binding or galling. This means the threads engage more fully, giving a consistent clamp load when tightened, better alignment, and reduced play between the bolt and its nut or hole. That tighter engagement helps prevent loosening under vibration and fatigue, which is crucial in flight hardware.

If the fit were looser, there would be more play and a higher risk of movement and looseness over time. If it were much tighter, assembly could become difficult, and thread damage could occur. So, Class 3 provides the right balance for aircraft bolts.