Definition
A machining or hand-finishing operation in which an existing hole is opened up to a precise final diameter, typically using a reamer or similar precision cutting tool. The hole is first drilled slightly undersize, then enlarged in a controlled second step so the finished diameter matches a specified tolerance.
Plain English
Making an existing hole bigger to an exact size. The hole is already there; this step trims it out to the precise diameter the drawing calls for.
Context Anchor
Seen in aircraft maintenance instructions, especially when fitting bolts, bushings, rivets, or other parts that must match a precise size.
Derivation
Dimension comes from a Latin word meaning “to measure out.” That fits this maintenance use: the important point is not just making something larger, but making it larger to a measured final size.
Why Pilots Care
Precision-sized holes matter for fit. A bolt hole that is slightly too small binds the bolt; one that is too large allows movement and wear. Holes for bushings and bearings must be exact so the part seats correctly and stays in place under load.
Intuition Check
Do not read this as “make it larger until it looks right.” It means “make it larger only to the exact measured size required.”
Example Sentence 1
After drilling the pilot hole, the mechanic used a reamer to enlarge it to an accurately specified dimension before installing the bushing.
Example Sentence 2
After measuring the worn bushing, the mechanic chose to enlarge it to an accurately specified dimension instead of ordering a new housing.