A hydraulic hose is a reinforced flexible tube that carries pressurised fluid between the pump, valves and cylinders in a hydraulic system. It has three layers: an inner tube that holds the fluid, a braided or spiral steel reinforcement that takes the pressure, and an outer cover that protects it from wear. Get the size, pressure rating and end fittings right and a hose lasts for years.
Quick answer
- Hydraulic hose is built in three layers: inner tube, steel reinforcement, outer cover.
- Dash size sets the bore in sixteenths of an inch, so a -8 hose is 8/16, or half an inch.
- Always match or exceed the pressure rating of the hose you replace.
- Measure the old hose end to end and identify both fittings before ordering.
How hydraulic hose is built
The inner tube is a synthetic rubber or thermoplastic that resists the hydraulic fluid. Around it sits the reinforcement, either one or two layers of braided wire for general pressure, or several spiral-wound layers for very high pressure. The outer cover shrugs off abrasion, oil and weather. More reinforcement layers mean a higher pressure rating, which is set by the hose standard, such as the common SAE 100R types.
Hydraulic hose sizes and dash sizes
Hydraulic hose sizes use a dash number that describes the inside diameter in sixteenths of an inch. A -4 hose has a quarter-inch bore, a -8 has a half-inch bore, and so on. The bore controls flow: too small and the fluid moves too fast, which heats the system and wastes power. Match the replacement hose to the original dash size unless you are deliberately re-engineering the flow.
| Dash size | Inside diameter | Typical use |
|---|---|---|
| -4 | 1/4 in (6.3 mm) | Gauges, pilot lines, small tools |
| -6 | 3/8 in (9.5 mm) | Light machinery, log splitters |
| -8 | 1/2 in (12.7 mm) | General mobile hydraulics |
| -10 | 5/8 in (15.9 mm) | Loaders, mid-size equipment |
| -12 | 3/4 in (19.1 mm) | Higher-flow main lines |
Hydraulic hose ends and fittings
The hose is only half the job; the ends decide whether it seals. Common hydraulic hose fittings include JIC, which seals on a 37-degree cone, BSP in British systems, NPT tapered threads, and ORFS, which seals on a flat face with an O-ring. These do not interchange. Fitting the wrong thread either leaks or damages the port, so identify both ends of the old hose before you order the new one.
How to measure and replace a hose
Measure the length from the very end of one fitting to the end of the other, not just the rubber. Note the dash size, the pressure rating printed on the cover, and the type and thread of each fitting, including whether the ends are straight or elbowed. With those four facts, bore, length, pressure and fittings, a replacement hose will drop straight in.
Relieve all pressure and lower any raised equipment before you crack a hydraulic fitting. Pinhole leaks under pressure can inject fluid through skin, which is a genuine medical emergency, so never search for a leak with a bare hand.