Aircraft bolts are usually manufactured with a which fit for the threads?

• A. class 1 fit
• B. class 2 fit
• C. class 3 fit
• D. class 4 fit

Answer: (C)

Thread fit classes describe how tightly the bolt threads are finished relative to the mating hole or nut. In aircraft, a tight, precise engagement is essential because joints endure vibration, loads, and temperature changes, and you want a predictable clamp force when the bolt is torqued. The closest, most controlled tolerance among the standard options is Class 3 fit. Using this tight fit ensures the threads engage with minimal clearance, giving consistent torque-tension behavior, reduced play, and better resistance to loosening under flight vibrations. Looser fits would introduce more clearance and variability in clamping force, which is undesirable in aircraft structures. So, aircraft bolts are typically manufactured with a Class 3 fit to achieve reliable, uniform engagement.