In the automotive world, hose size is expressed by inside diameter. For example, a 3/8 inch has has a nominal (approximate) inside diameter (ID) of 3/8 inch. Not so with aircraft hose. Aircraft hose uses dash numbers to designate the hose size.
Dash size is in 1/16 of an inch. So
-2 (2/16) 1/8 inch,
-6 (6/16) 3/8 inch,
-8 (8/16) 1/2 inch and so on.
But be careful, this does not mean that a -6 hose has an inside diameter of 3/8 inch. In fact, it can be anywhere from .298 to .359 inch, depending upon the hose type.
Common aircraft hose dash sizes are:
-2 Smallest hose size. Used as an instrument gage line
-3 Also used as an instrument gage line
-4 Common hydraulic line. Some smaller fuel line
-6 Common size for fuel delivery lines
-8 Fuel delivery and oil cooler lines
-10 Larger aircraft, oil and fuel
-12 Larger aircraft, oil and fuel
-16 Larger aircraft, oil and fuel
The inside diameter of each hose and fitting dash size is found in size charts. A common mistake is to think all hose of the same dash size has the same inside diameter. There not
A common mistake is to think that all fittings of the same dash size have the same inside diameter. They do not.
A common mistake is to think that a straight -6 fitting and a 90 -6 fitting of used on the same hose should both have the same inside diameter. They do not. A hose with a straight fitting on one end and an angle fitting on the other end will have different inside diameters.
To size aircraft hose and fittings you need to measure the item and compare its size to a size chart. Hose and fitting size charts are included in Mechanic's Toolbox Software.
The most full-proof method of determining what size hose assembly you have is to measure the thread diameter on the fitting and compare with the chart below. This chart will work with both AN and JIC fittings.