Definition
The largest diameter of a screw thread, measured across the outside of the threads (the crests on an external thread, or the roots on an internal thread). On a bolt, it is the diameter measured across the tips of the threads.
Plain English
The widest measurement across a threaded part, taken from one outside edge of the thread to the other.
Context Anchor
Seen in aircraft maintenance when checking threaded hardware, matching bolts and nuts, or reading thread-size specifications.
Derivation
From Latin major, meaning 'greater,' and diameter, from Greek diametros, 'measuring across.' Together it simply means 'the greater across-measurement' — the larger of the two diameters that describe a thread.
Why Pilots Care
Bolts and screws on an aircraft are specified by their major diameter (e.g., a 1/4-inch bolt). Using the wrong size hardware can cause fit, strength, or safety issues, so identifying parts correctly during inspection or replacement matters.
Analogy
Think of a threaded bolt as a tiny ridged cylinder. The major diameter is the measurement across the highest ridges, like measuring the widest part of a ridged pencil grip.
Intuition Check
Major does not mean “more important” here. It means the larger of the thread diameters being measured.
Example Sentence 1
The bolt's major diameter measured one-quarter inch, matching the specification in the maintenance manual.
Example Sentence 2
When matching a replacement fastener, measure both the major diameter and the thread pitch to avoid fit issues.