Definition
The measurement across the inside of a hollow cylindrical object, such as a tube, pipe, hose, bearing, or bushing, taken from one inner wall straight through the center to the opposite inner wall. Inside diameter defines the size of the open bore and is distinct from outside diameter, which measures across the outer wall.
Plain English
The width of the hole inside a round part — measured from one inner wall, through the middle, to the other inner wall.
Context Anchor
Seen in aviation maintenance when measuring parts that must fit together correctly, such as tubes, cylinders, holes, sleeves, and bearings.
Derivation
Diameter comes from Greek words meaning “measure across.” That helps here because an inside diameter is not a length along the part; it is the measurement across the opening through the center.
Why Pilots Care
Components like fuel lines, hydraulic hoses, and bearings must match the specified inside diameter exactly. The wrong inside diameter can restrict flow, leak, or fail to fit on a shaft — any of which can cause a system to malfunction.
Intuition Check
Do not confuse inside diameter with outside width. Inside diameter measures the open space within the part, not the full width of the part from outer edge to outer edge.
Example Sentence 1
The replacement hydraulic hose had the correct inside diameter, so the fluid flow rate matched the original specification.
Example Sentence 2
During overhaul the technician measured the inside diameters of the hydraulic cylinders with a bore gauge.