Hose dash sizes: −04 to −48, and the one grade where the number means something else

Dash sizes are the one piece of hydraulic shorthand that genuinely is simple: divide by sixteen and you have the bore in inches. The trouble is that people extend the shorthand to things it was never describing — the outside of the hose, the port thread, and one grade where the number is not measuring the bore at all.

/01The conversion.

Dash size is nominal bore in sixteenths of an inch. Nominal is doing real work in that sentence: it is the size the hose is called, and the actual bore is close to it rather than exactly it. DN is the metric nominal equivalent, and it is the figure European drawings and fittings use.

If you only want the conversion, our dash size chart gives it to four decimal places and nothing else. The rest of this page is about the three places where the conversion stops being the answer.

/02The R5 exception.

SAE 100R5 does not follow the rule above. It is sized on tube outside diameter — the size of the rigid tube the hose was designed to replace — rather than on its own bore. The result is that an R5 dash number sits one step out from everybody else’s for most of the range.

/03What the dash number does not tell you.

The most consequential misuse of a dash size is treating it as a description of the outside of the hose. It is not, and the spread is wide. Here is the same −08 bore across every construction we stock.

Nearly six millimetres between the slimmest and the fattest at an identical bore. That is the difference between a bundle that fits its clamp and one that does not, and it is invisible if you specify by dash size alone. Anything that touches the outside of the hose — clamps, sleeves, spiral guard, bulkhead penetrations, bundle spacing — has to be sized from the grade’s outside diameter. At −08, for example, 2SC measures 20.8 mm over the cover against 22.2 mm for 2SN — enough to decide whether a bundle fits its clamp.

/04Ordering without ambiguity.