Definition
A classification of screw threads based on the amount of tolerance and allowance permitted in their manufacture, which determines how tightly the threads of a bolt and nut fit together. In aviation hardware, common classes include Class 2 (a free-fitting thread used for general purposes) and Class 3 (a closer, tighter-fitting thread used where higher precision and strength are required).
Plain English
A grading system that tells you how loose or tight the fit is between a bolt's threads and the matching nut or hole. A higher class number means a tighter, more precise fit.
Context Anchor
Seen in aircraft maintenance when selecting, inspecting, or replacing bolts, screws, nuts, and threaded fittings.
Derivation
Class comes from a word meaning a group or category. Thread originally referred to a thin strand, and later came to mean the spiral ridge on a screw because it winds around the part like a strand wrapped around it.
Why Pilots Care
Using the wrong class of thread can mean a fastener that is too loose to hold reliably or too tight to seat properly. On an aircraft, that can lead to a fastener backing out or failing under load, so maintenance personnel must match the specified class exactly.
Analogy
It is like choosing the right tightness of fit between a lid and a jar. Some fits are easy and loose; others are made to be very close and precise.
Intuition Check
Do not read class as school level or quality grade here. In this context, class means the specified category of thread fit.
Example Sentence 1
The maintenance manual specified a Class 3 thread for that engine mount bolt because the joint required a tight, precise fit.
Example Sentence 2
A Class 1 thread fit allows more play and is used where easy assembly matters more than tight tolerance.