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Fittings Guide
Understanding hose fittings
Almost every fitting problem is an identification problem. Two threads that look the same, measure nearly the same and will start together are not the same, and the joint that results leaks forever no matter how hard anybody tightens it.
- NPT is 60 degrees, BSP is 55. At half an inch they share 14 threads per inch, so they will start together.
- Only tapered pipe thread wants sealant. Tape on a flare or an O-ring fitting stops it sealing.
- JIC seals on a 37 degree cone, metal to metal, and nothing else.
- SAE ORB and ORFS seal on an O-ring, not on the thread. Check the O-ring every time.
- BSPP is parallel and will not wedge into an NPT female at all.
- Bring the fitting in. Thread gauges settle in seconds what guessing never will.
Where it sealsFive families, five different mechanisms
Before threads, understand that fittings do not all seal in the same place. Get this wrong and everything after it goes wrong too.
Sixty degree thread angle, tapered so it wedges as it tightens. It seals on the threads themselves, by interference, which is why microscopic gaps remain and why it is the one type that genuinely wants sealant.
Sealant yesStraight thread with a 37 degree cone. The seal is metal to metal where the two cones meet, and the thread does nothing but pull them together. The workhorse of hydraulics.
Never tapeStraight thread with an O-ring that seals against the flat face of the port. Excellent under vibration because nothing depends on thread interference. The O-ring is the seal, so it gets replaced.
Never tapeFlat faces with an O-ring in a groove between them. The best of the lot for vibration resistance and leak-free service, which is why it appears on serious mobile equipment.
Never tapeFifty-five degree Whitworth thread with rounded crests. Comes in two forms: BSPP is parallel and seals on a bonded washer or O-ring, BSPT is tapered and seals by interference like NPT.
BSPT onlyThe pattern is simple once you see it. Tapered pipe threads seal on the thread and want sealant. Everything else seals on a face or a cone and must not have anything on the thread.
The classic trapNPT and BSP
They will start. They will not seal.
NPT uses a 60 degree thread angle with pointed crests and roots. BSP uses a 55 degree Whitworth profile with rounded ones. Five degrees, and they are not interchangeable.
The reason people get caught is that at some sizes the threads per inch match exactly. At half an inch, both NPT and BSPT run 14 threads per inch, so a BSP fitting will thread into an NPT port and feel like it is going. It nips up, feels tight and weeps forever, and by then the port is damaged.
At that size the two are also well under a millimeter apart in outside diameter, so calipers alone will not resolve it. A thread gauge will, in about five seconds.
Two useful tells before you reach for a gauge. Look at the crests: NPT is pointed, BSP is rounded. And check for taper: BSPP is parallel, so it will not wedge into an NPT female at all and the mismatch shows immediately. BSPT tapers like NPT, which is the harder case.
BSP shows up constantly on European and Asian equipment. If a machine came from either, assume BSP until a gauge says otherwise. There is a clear side-by-side comparison from Brennan on the BSP and NPT difference, and Brennan is one of the lines we carry.
SealantWhere tape belongs and where it ruins things
This is the mistake we see most across the counter, and it is entirely avoidable.
- Tapered pipe thread. NPT and BSPT seal on the thread, so a thread sealant or PTFE tape fills the helical gap and does real work. Two or three wraps, in the direction the fitting turns.
- Flare fittings. JIC seals where two cones meet. Tape there holds the faces apart, which is the exact opposite of what you want, and it sheds shreds into the system.
- O-ring fittings. ORB and ORFS seal on an elastomer against a face. Tape on the thread changes nothing except the torque you need and the debris you introduce.
Stop tightening. On a flare or O-ring fitting, extra torque damages the sealing face and makes the next attempt worse. Check the seat angle, check the O-ring, check that the two halves are the same standard. A joint that needs force to seal is telling you something is mismatched.
MaterialsChoosing for where it lives
| Material | Suits | Watch for |
|---|---|---|
| Carbon steel | The default for hydraulic work. Strong, economical, compatible with petroleum fluids | Corrodes where it is wet or salted |
| Stainless steel | Marine, washdown, chemical and anywhere corrosion is the issue. 316 for genuinely marine work | Costs more, and galling on threads if assembled dry |
| Brass | Lower pressure work, air, water, instrumentation | Not a substitute for steel on hydraulic pressure |
| Bronze | Marine service where brass would suffer | Cost, and availability in some configurations |
On Long Island the deciding factor is usually salt rather than pressure. Anything on a boat, near the water or on a vehicle that sees winter roads should be stainless, and a plated steel fitting in that service will not last a season.
Identifying itWhat to do with an unknown fitting
Is it tapered or parallel?
Measure the outside diameter at two points along the thread. If it changes, it is tapered, which puts you in NPT or BSPT territory. If it is constant, you are looking at BSPP, JIC, ORB, ORFS or metric.
Is there a cone or a flat face?
A 37 degree cone means JIC. A flat face with an O-ring groove means ORFS. A straight thread with an O-ring behind the shoulder means SAE ORB. That single look eliminates most of the possibilities.
Are the crests pointed or rounded?
Pointed suggests NPT or metric, both 60 degree. Rounded suggests BSP at 55 degree. It is not conclusive on a worn fitting but it narrows things quickly.
Then use a gauge
Thread pitch gauge for the pitch, thread ID gauge for the angle, calipers for the diameter. At the sizes where NPT and BSP overlap, the gauge is the only thing that settles it. Bring the fitting in and we will do this at the counter.
Worth noting that metric threads are also 60 degree and parallel, and their pitch is in whole or half millimeters rather than threads per inch. If the pitch gauge lands neatly on a metric value, that is your answer.
MistakesFive that cause most of the trouble
- Forcing a mismatched thread. If it needs effort to start, stop. Cross-threading a port turns a five dollar problem into a component replacement.
- Tape on a flare. Covered above, and it is the single most common one.
- Reusing a tired O-ring. On ORB and ORFS the O-ring is the seal. It costs almost nothing and it is the part that fails.
- Adapting instead of correcting. Adapters are legitimate and we sell plenty, but every extra joint is another leak path. If a system has grown a chain of three adapters, the right fix is usually a fitting that goes straight to the port.
- Assuming the replacement matches. Equipment gets repaired by other people. The fitting on the machine now is not necessarily the fitting the manufacturer specified.
CompatibilityBeyond the thread
A fitting that threads correctly can still be the wrong part.
- Pressure. The assembly is rated at whichever component is lowest. A high pressure hose on a lower rated fitting is a lower rated assembly.
- Fluid. The seal material has to suit what is going through it. O-ring compounds are not universal.
- Temperature. Ratings fall as temperature rises, and elastomer seals have a narrower window than the metal around them.
- Vibration. Where things shake, O-ring designs hold up far better than thread interference.
- Crimp pairing. If the fitting terminates a hose, it has to be from the same system as the hose. See crimping versus clamp-on and this note on hose and fitting compatibility.
At the counterBring it in
Thread identification is thirty seconds of somebody's time with a gauge, and it is the single most useful thing we do that people do not know to ask for.
- Tens of thousands of fittings across NPT, JIC, ORB, ORFS, BSP and metric.
- Adapters between standards, including the awkward international conversions.
- Identification while you wait. Bring the fitting, the hose, or the whole assembly.
- Steel, stainless and brass, chosen for where it lives rather than what is nearest.
- Crimped onto matched hose so the pairing is right. See hydraulic hose crimping.
Full range on hydraulic fittings and adapters and industrial fittings and adapters, plus quick disconnects and pipe fittings and valves.
Proudly serving Long Island since 1964
Sixty years identifying fittings across the counter for Long Island shops, fleets and contractors, from Nassau County and Suffolk County.
There is a reason people bring the part rather than describe it. A fitting in your hand can be gauged in seconds. A photograph, a phone description, or a part number off a machine that has been repaired twice usually cannot, and every wrong guess is another trip.
Common Questions
Hose fitting FAQ
NPT uses a 60 degree thread angle with pointed crests. BSP uses a 55 degree Whitworth profile with rounded crests. They are not interchangeable. The trap is that at some sizes the threads per inch match, so at half an inch both run 14 threads per inch and a BSP fitting will start in an NPT port, nip up, feel tight and weep forever.
Only on tapered pipe thread, meaning NPT and BSPT, because those seal on the thread itself. Never on JIC, which seals metal to metal on a 37 degree cone, and never on SAE ORB or ORFS, which seal on an O-ring against a face. Tape on those holds the sealing faces apart and sheds debris into the system.
Check whether it tapers by measuring the outside diameter at two points. Look for a cone or a flat face with an O-ring, which identifies JIC, ORFS or ORB immediately. Check whether the crests are pointed or rounded. Then confirm with a thread pitch gauge and a thread ID gauge, because at the sizes where NPT and BSP overlap only a gauge settles it.
BSPP is parallel and seals on a bonded washer or O-ring against a face. BSPT is tapered and seals by thread interference like NPT. Because BSPP is parallel it will not wedge into an NPT female at all, which makes the mismatch obvious. BSPT is the harder case because it tapers too.
Almost always a mismatch rather than insufficient torque. Check the seat angle, because a 37 degree flare threaded into a 45 degree seat nips up and weeps forever. Check the O-ring if it is an ORB or ORFS. And check both halves are the same standard. On a flare or O-ring fitting, extra torque damages the sealing face and makes the next attempt worse.
A straight thread with a 37 degree cone. The seal is metal to metal where the two cones meet, and the thread only pulls them together. It is the workhorse of hydraulic connection and it takes no sealant of any kind.
Carbon steel is the default for hydraulic work. Stainless where corrosion is the issue, and 316 for genuinely marine service. Brass for lower pressure air, water and instrumentation, not as a substitute for steel on hydraulic pressure. On Long Island salt usually decides it rather than pressure.
Adapters are legitimate and often the right answer, but every extra joint is another leak path and another thing to work loose. If a system has grown a chain of three adapters, the better fix is usually a single fitting that goes straight to the port.
Ask The Counter
Not sure what thread that is?
Send a photo of the fitting alongside a ruler, and tell us what equipment it came off. Better still, bring it in and we will gauge it while you wait.
Gauged, Not Guessed
Bring the fitting in
NPT, JIC, ORB, ORFS, BSP and metric, plus the adapters between them. Two Long Island counters, six days a week.