In hydraulic systems, the dash number, dash size or simply dash is an industry standard measuring system for hoses and fittings. If you are replacing hoses or fittings, you should choose the correct dash size to avoid damage from heat or turbulence.
The size of a hydraulic hose is described by its internal diameter, expressed as a dash number. The internal diameter of a hose measuring 1/4 of an inch can be divided into four segments, each measuring 1/16 of an inch, so the hose has a dash number of 4.
Dash is used, along with other configuration codes, to provide meaningful shorthand descriptions of hydraulic parts. The dash is a single number that precedes the coupling description of a hydraulic part and denotes the fitting size in sixteenths of an inch.
If a hydraulic adapter part is described as 6 MP -- 4 FPX 90, the numbers 6 and 4 are dash numbers; this part has a 6/16 (3/8) of an inch male connector on one end and a 4/16 (1/4) female swivel connector on the other, at right angles (90 degrees) to each other.