Demystifying Dash Sizes: What -4, -6, -8 Mean in Hydraulic Fittings

The Rule of Sixteenths

The system for dash numbers is simple and consistent: the dash number represents the size in sixteenths of an inch. This primarily refers to the nominal tube Outer Diameter (OD) or the hose Inner Diameter (ID). To find the size in inches, you simply divide the dash number by 16.

Common Dash Size Examples:

• -4 (four-dash) = 4/16" = 1/4"
• -6 (six-dash) = 6/16" = 3/8"
• -8 (eight-dash) = 8/16" = 1/2"
• -10 (ten-dash) = 10/16" = 5/8"
• -12 (twelve-dash) = 12/16" = 3/4"
• -16 (sixteen-dash) = 16/16" = 1"
• -32 (thirty-two-dash) = 32/16" = 2"

This system avoids fractions and provides a convenient shorthand that is used universally in part numbers and specifications.

How Dash Sizes Relate to Threads and Ports

While the dash size indicates the nominal tube or hose size, it also correlates to a specific thread size for each type of fitting. It is critical to remember that this correlation is **not** universal across different fitting types. A -8 JIC fitting and a -8 ORFS fitting will connect to the same 1/2" tube, but they have different thread sizes.

Examples of Dash Size and Thread Correlation:

• A -8 JIC fitting (for 1/2" tube) has a 3/4-16 UNF thread.
• A -8 ORB fitting (for a 1/2" port) also has a 3/4-16 UNF thread.
• A -8 ORFS fitting (for 1/2" tube) has a 13/16-16 UN thread.

This standardization within each fitting type is what allows for interchangeability between manufacturers who adhere to the same SAE or ISO standards. When someone refers to a "-8 port" on a hydraulic manifold, they are typically referring to an SAE standard port for a 1/2" line, which has a 3/4-16 UNF thread.

Dash Sizes for Hoses

For hydraulic hoses, the dash number almost always refers to the hose's Inner Diameter (ID). This is a crucial measurement for engineers calculating fluid velocity and flow rates. A "-12 hose" has a 3/4" inner diameter, capable of carrying a higher volume of fluid than a "-8 hose" (1/2" ID). The Outer Diameter (OD) of a hose can vary significantly based on its pressure rating and construction (e.g., 2-wire vs. 4-wire braid), so the dash size only reliably indicates the ID.

A Note on Metric and Pipe Threads

The dash size system is fundamentally based on inches. While metric tubing is sometimes referred to by its closest dash size equivalent (e.g., a 10mm tube is close to 3/8", or -6), this is only a colloquial reference. Metric systems have their own standards (like DIN 2353) based on tube OD in millimeters.

It is also important not to confuse a dash size with a nominal pipe thread size like NPT. A "-2 fitting" refers to a 1/8" tube size, whereas a "1/8 NPT fitting" refers to a specific pipe thread standard with its own unique dimensions (an actual OD of about 0.405"). The dash system primarily applies to standardized hose and tube connectors like JIC, ORB, and ORFS.

Conclusion

The hydraulic dash size system is a simple and effective industry shorthand for specifying the size of hoses, tubes, and their corresponding fittings. By understanding that the dash number equals the size in sixteenths of an inch, technicians, engineers, and purchasers can communicate clearly and ensure that all components in a hydraulic circuit are correctly matched. This common language simplifies ordering, reduces errors, and is fundamental to building and maintaining reliable fluid power systems.