A replacement hose can look right, screw on cleanly and still be the wrong specification. Undersize it and oil velocity rises, creating heat, pressure loss and sluggish machine operation. Oversize it unnecessarily and you add cost, weight and routing problems. Knowing how to size hydraulic hose means checking the complete working conditions, not simply matching the outside diameter of the old assembly.
For plant repairs, workshop equipment and mobile machinery, get the machine back to its original specification wherever possible. If the original hose has failed, do not assume its bore was the problem. Abrasion, poor routing, excess temperature, pressure spikes and incompatible oil can all cause a failure even where the hose size itself is correct.
Start with flow rate and hose bore
The hose bore, usually called the inside diameter or ID, controls the speed at which hydraulic oil travels through the line. It must be large enough to carry the required flow without excessive restriction, while remaining practical to route around the machine.
Flow is normally expressed in litres per minute (L/min). Hose size is commonly quoted in dash sizes, where one dash equals 1/16 inch of bore. For example, a -6 hose has a nominal 3/8 inch bore, while a -8 hose has a nominal 1/2 inch bore. Check the actual product specification, as construction and fitting style can affect the effective bore.
A useful sizing calculation is:
Hose ID in mm = √(Flow in L/min ÷ Velocity in m/s × 21.2)
You do not always need to calculate this by hand. A hydraulic hose flow chart will give a suitable bore from the known flow and intended oil velocity. The key is using the correct velocity for the job.
Use the right oil velocity
There is no single perfect velocity for every hydraulic line. Pressure lines can tolerate a higher velocity because they are generally shorter and carry oil under pump pressure. Return lines need more care because excessive restriction can increase back pressure and affect the return side of the system. Suction lines need the lowest velocity of all, as a restriction at the pump inlet can cause cavitation.
As a practical guide, many systems are designed around these ranges:
- suction lines: approximately 0.6 to 1.2 m/s
- return lines: approximately 2 to 4.5 m/s
- pressure lines: approximately 3 to 6 m/s
Size hydraulic hose for pressure, not just flow
Once you have selected a likely bore, check that the hose pressure rating is suitable for the circuit. The hose working pressure must meet or exceed the maximum working pressure of that line. Do not select hose based solely on the normal pressure shown on a gauge. Hydraulic systems can experience short, sharp pressure spikes when a valve closes, a cylinder reaches the end of its stroke or an attachment is suddenly stopped.
The pressure rating of an assembly is limited by its lowest-rated component. A high-pressure hose with a lower-rated coupling does not make a high-pressure assembly. The hose, fittings, adaptors and any quick-release couplings must all be appropriate for the system pressure.
Look for the hose standard and rating rather than relying on appearance. Two hoses with the same bore can have very different reinforcement, bend radius and working pressure. A textile-braided low-pressure return hose is not interchangeable with a wire-braided pressure hose, even if both are 1/2 inch bore.
For a machine repair, identify whether the line is suction, return, medium-pressure or high-pressure before ordering. If the original marking is readable, record the hose type, dash size, working pressure and any applicable SAE or EN standard. This provides a much safer starting point than measuring the outside diameter alone.
Check the hose length and routing
Correct bore and pressure rating will not compensate for a hose that is too short, sharply bent or allowed to rub against the chassis. Measure the old hose assembly from sealing face to sealing face, not from the ends of the fitting nuts. If the old hose has stretched, failed or been routed incorrectly, measure the intended path on the machine instead.
Allow sufficient length for movement. A hose connected to a loader arm, excavator boom, tipping body or steering axle must accommodate the full range of travel without being pulled tight. It also needs enough slack to avoid being crushed, pinched or caught on moving parts.
The minimum bend radius shown for the hose is critical. Bending a hose tighter than its rating can damage reinforcement layers and restrict oil flow. The bend should begin behind the fitting, not immediately at the ferrule. If space is tight, a swept elbow fitting or a different routing arrangement is usually better than forcing a tight bend into the hose.
Avoid twisting the hose during installation. A twisted pressure hose is under stress before the machine has even started. Use the layline or printed marking along the hose as a visual guide. If the line spirals after fitting, loosen and realign it before putting the system under pressure.
Identify the fittings properly
Fitting identification causes a large share of incorrect hydraulic hose orders. Thread size alone is not enough. Hydraulic connections can have similar dimensions but use different sealing methods, including BSP, BSPT, JIC, ORFS, metric, NPT and flange connections.
Check the thread diameter and pitch, then identify where the seal is made. A BSPP fitting may seal with a bonded seal or washer. A JIC fitting seals on a 37-degree flare. An ORFS fitting seals with an O-ring on a flat face. These designs are not interchangeable, and forcing the wrong fitting together can damage threads or create a dangerous leak.
Record the fitting type, thread size, orientation and hose end configuration. For example, a straight fitting at one end and a 90-degree swept fitting at the other must be measured for orientation. Where both ends are angled, the assembly may need to be clocked so the fittings align correctly with the machine ports.
If you are replacing a hose with quick-release couplings, verify the series, thread connection, pressure rating and whether the circuit requires a flat-face coupling to reduce oil loss and contamination. Couplings that connect physically are not automatically compatible in flow capacity or operating pressure.
Consider fluid, temperature and environment
Hydraulic oil is not the only fluid carried by hose. Some machines use biodegradable oils, water glycol fluids, fuel, compressed air or specialised fluids that need a compatible tube material. Check the hose manufacturer's fluid compatibility information before selecting a replacement.
Temperature matters on both sides of the hose. The internal temperature is determined by the fluid, while the external temperature may come from an engine bay, exhaust, welding work or hot surfaces. High temperature accelerates hose ageing, while very cold conditions can reduce flexibility and make routing more difficult.
External damage is equally important. Plant and agricultural equipment often needs abrasion-resistant hose covers or protective sleeves where lines pass near frames, tracks, sharp edges and moving attachments. Sleeves protect against wear, but they do not cure a badly routed hose. Secure the line with suitable clamps, leaving enough movement where the hose needs to flex.
A practical checklist before ordering
Before specifying a new assembly, confirm the circuit function, required flow, nominal bore, maximum pressure, hose length, fluid type and operating temperature. Then identify both fittings in full, including thread, seal type, angle and orientation. Finally, consider bend radius, movement, abrasion and any guards or clamps needed for the route.
If you have the original assembly, take clear measurements and photographs before removing it. Keep the failed hose until the replacement is confirmed. Its markings, fitting style and routing can answer questions that are easily missed once the machine is stripped down.
Do not guess when safety is involved
Hydraulic oil escaping from a pinhole leak can penetrate skin and requires urgent medical treatment. Never inspect a pressurised hose with your hand, and never use tape, clamps or makeshift repairs on a failed high-pressure line. Depressurise the system correctly, isolate the machine and fit a properly specified replacement assembly.
DYNATEX stocks hydraulic hose and couplings for practical repair and maintenance work, but the final specification must suit the machine and circuit. Take the extra few minutes to check flow, pressure, fittings and routing. A correctly sized hose is not just a tidier repair - it keeps the machine working efficiently and gives the operator one less failure to deal with on site.