Definition
The measurement across a hole, taken in a straight line that passes through the center, from one inner wall to the opposite inner wall. It defines the size of the opening itself, not the material surrounding it.
Plain English
How wide the opening is, measured straight across the middle from one inside wall to the other.
Context Anchor
Seen in aircraft maintenance drawings, inspection limits, and repair instructions when checking whether a bolt, pin, or other part will fit correctly in a hole.
Derivation
Diameter comes from the Greek 'diametros,' meaning 'measuring across.' 'Inside' specifies that the measurement is taken on the inner surface of the hole, distinguishing it from outside diameter, which is measured across the outer surface of a part such as a pipe or shaft.
Why Pilots Care
Correct inside diameter ensures parts fit securely and prevents structural weakness.
Analogy
It is like measuring the width of a straw opening from inner edge to inner edge, straight across the center.
Intuition Check
Do not read this as the depth of the hole or the distance around the hole. Inside diameter means the straight inside width across the center.
Example Sentence 1
The mechanic checked that the inside diameter of the hole in the bushing matched the outside diameter of the bolt before installation.
Example Sentence 2
If the inside diameter of the hole grows too large, the joint loses strength and must be repaired.