First, What Is an O-Ring Boss (ORB) Fitting?

An SAE O-Ring Boss (ORB) fitting is a smart, modern way to seal a port on a pump, cylinder, or valve. Think of it as the professional upgrade to an old-school pipe thread (NPT). Instead of relying on brute force to jam threads together, the ORB system uses a dedicated O-ring to do the sealing and straight threads just for clamping. It’s a simple, tough, and incredibly reliable system defined by the SAE J1926 standard.

The Nuts & Bolts: How an ORB Seal Actually Works

The brilliance of the ORB system is that it separates the job of holding from the job of sealing. It’s a two-part system:

• Straight Threads for Muscle: The UNF straight threads on an ORB fitting have one job: provide the clamping force to hold the fitting securely in the port. They don't do any sealing at all.
• The O-Ring for the Seal: A tough O-ring sits in a specially designed groove at the base of the male threads. When you tighten the fitting, that O-ring is compressed into a machined chamfer in the female port (the "boss"). This captured seal is what makes the connection leak-proof, even with the vibration that would kill a pipe thread connection.

ORB Sizing & Thread Chart (SAE J1926)

ORB fittings use the same straight thread dimensions and dash sizes as JIC 37° flare fittings. The key difference is the sealing method. An ORB fitting has that tell-tale O-ring at the base. Here is the standard O-Ring Boss size chart:

Dash Size	Thread Size (UNF)	O-Ring Size (AS568A Std.)
-02	5/16-24	902
-03	3/8-24	903
-04	7/16-20	904
-05	1/2-20	905
-06	9/16-18	906
-08	3/4-16	908
-10	7/8-14	910
-12	1-1/16-12	912
-16	1-5/16-12	916
-20	1-5/8-12	920
-24	1-7/8-12	924
-32	2-1/2-12	932

The Real World: The Good, the Bad, and How to Not Screw It Up

Why ORB is a Huge Step Up From Pipe Thread:

• It Stops Leaks. Period. The captured O-ring is far more reliable than a tapered thread, especially in systems with vibration and pressure spikes.
• It's Reusable: Because the threads aren't being deformed to make a seal, you can remove and reinstall an ORB fitting multiple times. Just pop a new O-ring on it and you're good to go.
• No Gunk in Your System: You don't need pipe dope or Teflon tape. That means no risk of sealant breaking loose and clogging a screen or valve downstream.
• You Can't Wreck the Port by Overtightening: While you should always use a torque wrench, the design is much more forgiving than a tapered thread. It's tough to crack a port with an ORB fitting.

What to Watch Out For:

• The O-Ring is Everything: The entire seal depends on that little rubber ring. It has to be the right size for the port and it can't be nicked, cut, or twisted. A ten-cent O-ring is protecting a fifty-thousand-dollar machine. Treat it like it.
• The Port Has to Be Right: You can't just drill and tap a hole. An ORB fitting needs a proper port machined to the SAE J1926 standard with the correct chamfer to accept the O-ring.

How to Install It Like a Pro

1. Wipe it clean. The port, the fitting threads, and the O-ring groove.
2. Check the O-ring. Make sure it's the right one and it's not damaged.
3. Lube the O-ring with a little clean system fluid. This helps it seat without getting pinched.
4. Thread it in by hand until the O-ring makes contact with the port face and the fitting is snug.
5. Use a torque wrench. Tighten to the spec for that size fitting. Don't guess. This ensures the O-ring is perfectly compressed—not crushed, not too loose.

For adjustable fittings like elbows and tees, you thread it in, back it out (never more than 360 degrees) to get your alignment, then hold the body still and tighten down the jam nut and washer.

The Bottom Line

The SAE O-Ring Boss is the industry standard for sealing ports on hydraulic components for a good reason. It’s tough, it’s reliable, and it solves the problems that have plagued pipe threads for decades. When you start with a quality fitting made to the J1926 spec and install it with a little care, you get a connection you never have to worry about again.