If you have ever tightened a hydraulic fitting until it groaned and watched it keep weeping, there is a good chance you were fighting an O-Ring Boss fitting and treating it like a tapered pipe thread. They are not the same thing, and the difference is the whole reason ORB exists.
What BOSS / ORB actually means
ORB stands for O-Ring Boss. You will also see it written as SAE J1926 or simply "BOSS." The fitting has a straight thread — not tapered — and a machined flat face, called the boss, at the base of the thread. An o-ring sits against that face.
The seal is made by the o-ring being compressed between the fitting shoulder and the boss face. The threads do no sealing at all. They exist only to pull the two faces together.
Why that matters when you are working on one
- Overtightening does not fix a leak. If an ORB fitting weeps, the o-ring is damaged, wrong-sized, or missing. More torque just distorts the boss face and makes the next repair harder.
- Sealant and tape do nothing. PTFE tape on a straight thread is at best useless and at worst sheds into the hydraulic circuit.
- The o-ring is not optional. An ORB fitting without its o-ring will leak no matter how it is torqued.
How ORB differs from the alternatives
NPT (tapered pipe thread) seals on thread interference and does need sealant. ORFS / FACE seal puts the o-ring in a groove on a flat face at the tube end — see our ORFS guide. Flange fittings use a split clamp and a captured o-ring for larger bores — see our flange guide.
ORB o-ring sizes: the 900 series
ORB uses the SAE 900 series dash numbering, which is specific to boss seals and does not map one-to-one onto the AS568 standard sizes you may know from general o-ring work. Dash 4 is #904, dash 6 is #906, and so on. Guessing across the two systems is how people end up with a ring that almost fits.
Stop hunting for one o-ring
The practical answer for anyone who maintains hydraulic equipment is to keep the common sizes on the shelf. Our Hydraulic O-Ring Kit for BOSS ORB Fittings contains 245 pieces across 12 common sizes in Buna-N 90 durometer, in a compartmented case with a sizing chart.
Contents: 35× dash 4 (#904), 30× dash 5 (#905), 30× dash 6 (#906), 20× dash 7 (#907), 20× dash 8 (#908), 20× dash 9 (#909), 15× each of dash 10 (#910), dash 11 (#911), dash 12 (#912), dash 13 (#913), dash 14 (#914) and dash 16 (#916).
90 durometer is the correct hardness for hydraulic pressure. A softer 70 durometer ring will extrude into the clearance gap and fail early under the same conditions.
Before you reassemble
Every o-ring in these kits seats better and lasts longer with a food-grade silicone lubricant. Dry o-rings pinch, twist and take a set, which is how a brand-new seal starts weeping within a season. A thin film is all you need.
Aqua Flex Food Grade Silicone Lubricant is what we use and what ships in our kits. Apply it to o-rings only — never to the polished faces of a mechanical shaft seal. Those faces need to run dry and clean against each other, and lubricant on the carbon or ceramic will cause the seal to slip, overheat and leak.
Beaver Island Parts Company is not affiliated with, endorsed by, or sponsored by any equipment manufacturer named on this page. All brand names, model numbers and part numbers are the property of their respective owners and are used here solely to identify the equipment our replacement parts fit.