O-Ring Boss & Face Seal Fitting Seals
ORB and ORFS hydraulic fitting O-rings — correct size, material and torque for leak-free SAE ports.
O-ring boss (ORB) and O-ring face seal (ORFS) fittings are the dominant leak-free connection systems in mobile and industrial hydraulics. Both use an elastomeric O-ring as the primary seal element, but the groove location, assembly visibility and pressure capability differ. Specifying the wrong size or confusing ORB with ORFS is a leading cause of field leaks. This application page summarizes **where O-rings seal in ORB and ORFS systems**, **which materials and hardness grades to stock**, and **field service rules** — with full technical detail in our [ORB & ORFS fittings guide](/blog/o-ring-boss-orb-orfs-fittings-guide/). **ORB (SAE J1926 / ISO 6149)** — the O-ring sits in a groove on the male stud and compresses against the flat floor of the female port counterbore. Straight threads do not seal; only the O-ring creates the pressure boundary. **ORFS (SAE J1453 / ISO 8434-3)** — the O-ring sits in a groove on the flat face of the fitting and compresses between two parallel metal faces. Higher typical pressure rating and excellent vibration resistance; O-ring is visible before assembly. Related: [hydraulic cylinders](/applications/hydraulic-cylinders/), [hydraulic fluid compatibility](/blog/o-ring-hydraulic-fluid-compatibility/), [NBR hydraulic seals](/products/nbr/hydraulic-seals/), [static flange seals](/applications/static-flange-seals/).
Application Requirements
Recommended Materials
NBR 90 Shore A
Default for mineral-oil hydraulic ORB and ORFS service ports. Higher hardness resists extrusion in -006 to -012 dash sizes.
Temp: −40°C to +120°C
Black NBR 90A is the industry standard for SAE ORB port seals. Do not use in phosphate ester (Skydrol) — specify FKM.
FKM 75–90 Shore A
Phosphate ester aircraft and fire-resistant hydraulic fluids, high-temperature zones, and aggressive synthetic oils.
Temp: −20°C to +200°C
Required for Skydrol and HyJet systems. Verify compound grade with fluid manufacturer.
HNBR 90 Shore A
Modern mobile hydraulics with biodiesel contamination, higher temperature at the port, and improved abrasion resistance versus NBR.
Temp: −30°C to +150°C
Growing OEM preference for construction and agricultural equipment ORB ports.
Design Tips
- 1.Replace the O-ring every time an ORB or ORFS fitting is disassembled — reuse is the most common cause of repeat leaks.
- 2.Lubricate with system-compatible hydraulic oil before assembly. Never use petroleum grease on EPDM or silicone port seals.
- 3.Torque to manufacturer specification in a cross pattern — uneven torque on ORFS face fittings creates high spots that cut the O-ring.
- 4.Inspect port counterbores (ORB) for scratches and burrs on the flat sealing floor — a damaged counterbore defeats a new O-ring.
- 5.Do not mix ORB and ORFS fittings on the same port geometry — the groove dimensions and sealing surfaces are not interchangeable.
- 6.For ORFS, verify face Ra ≤ 0.8 µm — rough faces cause weeping leaks even with a new O-ring.
- 7.Size ORB O-rings to SAE J1926 dash tables — field measurement of the removed seal is the reliable backup when port size is unknown.
- 8.Stock separate NBR and FKM ORB kits if your shop services both mineral oil and phosphate ester equipment.
Common Sizes
| Size | Typical Use |
|---|---|
| AS568 -006 (1.78 mm CS) | Small ORB ports — #4 and #6 SAE straight thread fittings |
| AS568 -008 to -012 | Most common mobile hydraulic ORB port sizes (#8, #10, #12) |
| AS568 -110 to -120 (2.62 mm CS) | Medium ORB ports and some ORFS face grooves |
| AS568 -214 to -218 | Large ORB ports on cylinders and manifolds |
| ORFS face groove (per SAE J1453) | Size varies by fitting series — match to fitting part number, not generic dash |
Frequently Asked Questions
What O-ring size fits an SAE ORB port?
ORB port O-ring sizes are defined in SAE J1926 by port size (e.g., -8, -12, -16). Each maps to a specific AS568 dash number. The most common mobile sizes are -008 through -012 for small ports and -110 through -120 series for medium ports. Always verify against the port identifier stamped on the manifold or use the dash table in our ORB fittings guide.
ORB or ORFS — which fitting type do I have?
ORB: straight thread port with internal counterbore — O-ring hidden on the stud after assembly. ORFS: flat-faced fitting with visible O-ring in a face groove before tightening. ORFS uses a nut that compresses the flat faces; ORB uses a threaded stud torqued into a port. See the comparison table in our ORB & ORFS blog guide.
Can I reuse an ORB O-ring after removing a fitting?
No. The O-ring takes permanent set and may have micro-cuts from the port edge during removal. Reinstalling a used ORB O-ring is a common cause of immediate weeping leaks under pressure. Always install a new O-ring and lightly lubricate with clean hydraulic fluid.
NBR or FKM for ORB hydraulic fittings?
NBR 90 Shore A for petroleum-based hydraulic oil — this is the default for 95% of industrial and mobile equipment. FKM is mandatory for phosphate ester fluids (Skydrol, HyJet) and recommended for continuous temperatures above +120°C at the port.
Why does my ORB fitting leak after I rotated it for orientation?
ORB fittings allow orientation adjustment, but the O-ring must be replaced if the stud was fully torqued and then backed off. Backing off breaks the compression set contact pattern. Loosen, replace the O-ring, seat lightly, rotate to position, then torque to final spec.
What torque for ORB hydraulic fittings?
Torque depends on port size, thread, and material (steel vs stainless). Typical range: 20–40 N·m for 7/16-20 UNF (#8) ports; 40–80 N·m for 3/4-16 UNF (#12) ports. Over-torque extrudes the O-ring; under-torque leaves insufficient compression. Always use the equipment manufacturer's torque table — do not guess.
Do you supply ORB and ORFS O-ring kits?
Yes. We supply NBR 90A and FKM ORB assortments in -006 through -120 dash sizes, plus custom kits matched to your equipment BOM. Specify whether you service mineral oil, phosphate ester, or mixed fleets. MOQ from 1 piece; kit MOQ from 10 sets.
Need ORB or ORFS O-rings?
Tell us the port size (SAE dash or fitting part number), fluid type and temperature. We match AS568 sizes and supply NBR, HNBR and FKM with batch CoC. MOQ 1 piece.