51mm

51 millimeters (51 mm) is a unit of length in the metric system equal to 0.051 meters or 5.1 centimeters. It is commonly used to describe dimensions where precision to the millimeter matters, such as diameters, thicknesses, or gaps in manufactured parts. In inches, 51 mm is approximately 2.01 inches.

Applications: In mechanical engineering and manufacturing, 51 mm can denote bore diameters, shafts, external housings, or component clearances. Dimensions are typically specified with tolerances under standards such as ISO, DIN, or ASME, which define permissible deviations from the nominal size. As a metric dimension, 51 mm is part of the broader practice of decimal-based measurement that facilitates interoperability in global supply chains.

Context in optics and photography: Lenses, filters, and mounts often use diameters measured in millimeters. A 51 mm dimension might appear as a thread or outer diameter specification for some components, reflecting the broader use of millimeters to describe physical sizes in these fields.

Other uses: The term 51 mm is primarily a length; when expressing the quantity, it is common to write 51 mm with a space between the number and unit, following general style guides in scientific writing.