What grade are aviation bolts typically made of?

The threads of aircraft bolts generally use Grade 3 precision, which corresponds to a medium fit—this is the standard choice for most aircraft bolts.

Why Level 3?

In the aerospace field, bolts not only need to withstand high-intensity loads but also balance assembly efficiency and reliability. Grade 3 precision (medium fit) strikes precisely this balance between these two aspects:

High connection reliability
The Class 3 threaded fit is tighter and can effectively prevent loosening caused by vibration and temperature changes, ensuring flight safety.

Good assembly operability
Compared to Class 4 or Class 5 interference fits (which require a wrench to be forcibly tightened), Class 3 bolts can be smoothly screwed in by hand, reducing installation difficulty and the risk of damage.

Comparison of Different Precision Levels
Level Coordination Type Feature Description Application Scenarios
Level 1: Loose Fit – Can be easily twisted with just your fingers – Used very rarely
Level 2: Free fit with a certain clearance for quick installation; commonly used for aviation screws.
Level 3: Moderate Compatibility—Balancing Tightness and Operability—Most Aviation Bolts
Grade 4-5: Tight fit—requires forceful tightening with tools, easily damages threads—suitable for high-load special applications, not mainstream.