Definition
An aircraft bolt machined to extremely precise dimensions, designed to fit its hole with little or no clearance. Close-tolerance bolts are used in locations that carry severe shear, vibration, or alternating loads, where any looseness between the bolt and the hole would cause the joint to wear or fail. They are typically driven into the hole with light taps from a 12- to 14-ounce hammer rather than slipping in by hand, and are identified by a triangle stamped on the head.
Plain English
A bolt made to such tight measurements that it fits snugly into its hole with almost no gap. It must be tapped gently into place, and it is used where the joint takes heavy or shifting loads.
Context Anchor
Seen in aircraft maintenance manuals, repair drawings, and logbook descriptions for structural joints where the bolt fit is critical.
Derivation
Tolerance comes from the Latin tolerare, meaning to bear or allow. In manufacturing, tolerance refers to the small amount of variation allowed in a part's dimensions. Close-tolerance therefore means the allowed variation is very small, so the bolt is made to a very precise size.
Why Pilots Care
Maintains exact alignment and load transfer in critical joints so the airframe does not loosen or shift under flight loads.
Analogy
It is like using a peg that fits a hole exactly instead of one that is slightly loose. The exact fit keeps the parts from shifting against each other.
Intuition Check
Close does not mean nearby here. Tolerance does not mean patience or acceptance; it means the small amount a manufactured part is allowed to vary from the exact size.
Example Sentence 1
The mechanic identified the close-tolerance bolt by the triangle stamped on its head and tapped it into place with a light hammer.
Example Sentence 2
During the inspection the IA verified that all close-tolerance bolts on the engine mount remained snug with no detectable play.