Definition 1 of 2
Definition
The threads inside a tapped hole or nut that engage with the threads of a bolt or screw. Fill threads are the female (internal) threads that the male (external) threads of the fastener fill when it is screwed in.
Plain English
The grooves cut on the inside of a hole or nut. When you screw a bolt in, its outside threads slide into and fill these inside grooves to hold everything together.
Context Anchor
Seen in aircraft maintenance when inspecting bolts, screws, nuts, tapped holes, and threaded fittings.
Derivation
Called 'fill' threads because the bolt's external threads slide in and fill the matching grooves cut into the hole or nut. The fastener literally fills the empty space.
Why Pilots Care
Correct orientation of fill threads ensures the fabric has the strength needed to withstand flight loads.
Intuition Check
Do not read “fill threads” as thread sealant or material used to fill a gap. Here it means the complete, usable threads that can properly engage and hold.
Example Sentence 1
Before installing the bolt, the mechanic inspected the fill threads in the tapped hole for damage or corrosion.
Example Sentence 2
A tear running parallel to the fill threads required a different patch technique than one along the warp.