AN vs NPT vs ORB vs BSP
Thread Standards
Crossed threads, weeping joints, and seized adapters – almost all caused by using the wrong thread standard. Learn to identify every type and connect them correctly.
Why Thread Standards Matter
Unlike bolts – where you can often mix thread standards if the pitch is similar – fluid-carrying fittings rely on precise thread geometry for their leak-free seal. Mixing incompatible thread types is one of the most common mistakes in performance plumbing, and the consequences range from a persistent seep to a catastrophic failure under load.
The key variables that define a thread standard are:
Form
Thread angle – the shape of the individual thread tooth. AN/JIC uses 60°. BSP uses 55° (Whitworth). This alone makes AN and BSP threads physically incompatible even when the pitch looks similar.
Taper vs Straight
Tapered threads (NPT, BSPT) seal by the threads themselves wedging and deforming. Straight threads (AN, ORB, BSPM) seal via a separate mechanism – a flare seat, O-ring, or crush washer.
Pitch & Diameter
Even if the form is the same, different diameter/pitch combinations mean fittings that appear to thread together are not properly engaged and will leak or back out.
AN / JIC Thread – SAE J514
The Army-Navy (AN) standard, also called JIC (Joint Industry Council) or SAE J514, is the foundation of performance fluid plumbing. AN threads are UN/UNF (Unified National Fine) straight threads – they do not seal by threading together, they simply engage the bodies mechanically. The seal is made by the 37° flare angle on the mating faces.
| Dash | Thread (UNF) | Thread O.D. | TPI | Hex (Nut) | Hex (Male Body) |
|---|---|---|---|---|---|
| -3 | 3/8-24 | 9.5mm | 24 | 9/16" | 7/16" |
| -4 | 7/16-20 | 11.1mm | 20 | 5/8" | 1/2" |
| -6 | 9/16-18 | 14.3mm | 18 | 11/16" | 9/16" |
| -8 | 3/4-16 | 19.0mm | 16 | 7/8" | 3/4" |
| -10 | 7/8-14 | 22.2mm | 14 | 1-1/16" | 7/8" |
| -12 | 1-1/16-12 | 26.9mm | 12 | 1-1/4" | 1-1/16" |
| -16 | 1-5/16-12 | 33.3mm | 12 | 1-1/2" | 1-5/16" |
| -20 | 1-5/8-12 | 41.3mm | 12 | 1-7/8" | 1-5/8" |
AN and JIC 37° threads are mechanically interchangeable – the thread form and dimensions are identical (both UN/UNF, 37° flare seat). An AN-6 swivel nut will correctly mate to a JIC -6 male body. The terms are used interchangeably in the aftermarket.
The 37° Flare Seal
When the swivel nut draws the convex 37° nose of the hose end (or male fitting) into the concave 37° female seat, the two machined aluminium (or steel) surfaces are compressed together to form a metal-to-metal seal. No thread sealant, no O-ring, no PTFE tape – the geometry is the seal. This is why nick-free, clean flare seats are critical. A single piece of swarf in the seat destroys the seal.
NPT – National Pipe Taper
NPT (National Pipe Taper, ANSI/ASME B1.20.1) is an American standard tapered thread. Both male and female halves are tapered at 1° 47' (approximately 1:16) – meaning the thread OD grows 1.79mm per 25.4mm of thread length. When the tapered male engages the tapered female, the threads mechanically wedge together and deform, creating the seal. A thread sealant (PTFE tape or anaerobic compound) fills any remaining voids.
| NPT Size | Actual O.D. | TPI | Engagement (turns) | Common Application |
|---|---|---|---|---|
| 1/8" NPT | 9.7mm | 27 | 2.5–3.5 | Sensor ports, oil pressure senders |
| 1/4" NPT | 13.7mm | 18 | 2.5–3.5 | Oil filter bosses, turbo oil feed (most common) |
| 3/8" NPT | 17.1mm | 18 | 3–4 | Oil drain ports, thermostat housings |
| 1/2" NPT | 21.3mm | 14 | 3–4 | Large oil ports, fuel tank bungs |
| 3/4" NPT | 26.7mm | 14 | 3–4 | Block drains, large coolant ports |
| 1" NPT | 33.4mm | 11.5 | 3.5–4.5 | Large manifold ports, dry sump systems |
NPT threads cannot be disassembled and reused indefinitely – each torque cycle progressively deforms the threads. Plan NPT connections as semi-permanent. For frequently-serviced ports (sensors, bleed screws), prefer ORB or AN-thread alternatives.
NPSM – National Pipe Straight Mechanical
NPSM uses the same thread form and pitch as NPT (60° thread angle), but without the taper – the thread OD is constant (straight). Because NPSM is straight, it cannot seal by thread wedging. Sealing is achieved by a sealing face, O-ring, or gasket at the thread shoulder. NPSM male threads can engage NPT female ports (due to the same pitch) but the seal will be unreliable – never substitute NPSM in an NPT port for a critical fluid-carrying application.
ORB – O-Ring Boss (SAE J1926)
O-Ring Boss (ORB), standardised under SAE J1926, is a straight-thread port connection that seals via an O-ring that is compressed into a machined counterbore at the base of the female port. The thread only serves to retain the fitting – 100% of the sealing load is carried by the O-ring.
ORB is the preferred port style for:
- ?Hydraulic systems (construction equipment, power steering, industrial)
- ?OEM engine oil ports on modern engines
- ?Automatic transmission oil cooler ports
- ?Power steering gear boxes and rack-and-pinion units
- ?Inline filter housings
| ORB Port Size | Thread | O-Ring Material | Sealing Method | Working Pressure |
|---|---|---|---|---|
| -4 ORB (1/4") | 7/16-20 UNF | Buna-N or Viton | O-ring in counterbore | 6,000 psi |
| -6 ORB (3/8") | 9/16-18 UNF | Buna-N or Viton | O-ring in counterbore | 5,000 psi |
| -8 ORB (1/2") | 3/4-16 UNF | Buna-N or Viton | O-ring in counterbore | 4,000 psi |
| -10 ORB (5/8") | 7/8-14 UNF | Buna-N or Viton | O-ring in counterbore | 3,500 psi |
| -12 ORB (3/4") | 1-1/16-12 UNF | Buna-N or Viton | O-ring in counterbore | 3,000 psi |
ORB and AN-6, AN-8, AN-10 share the same UNF thread pitch – a -6 AN male will thread into a -6 ORB port. However, the seal geometry is different: AN needs a 37° flare face, ORB needs an O-ring counterbore. Always use the correct fitting for the port type – cross-connection may appear to thread in but will not seal correctly.
BSP – British Standard Pipe
BSP threads (ISO 228 for BSPP / ISO 7 for BSPT) are used on most Japanese and European OEM fluid systems – engine blocks, gearboxes, air compressors, and hydraulic systems. There are two variants:
BSPP (Parallel / G-thread)
Straight thread, 55° thread form. Seals via a bonded seal washer (Dowty washer), copper crush washer, or O-ring face. The most common in Japanese automotive applications (Honda, Toyota, Nissan oil ports). Designated as G 1/4, G 3/8, etc.
BSPT (Tapered / R-thread)
Tapered at 1:16 – same taper ratio as NPT. Not the same as NPT – 55° vs 60° thread form. Seals by thread wedging + sealant (same concept as NPT). Designated as R 1/4-28, R 3/8, etc.
| BSP Size | Actual O.D. | TPI | Sealant? | OEM Examples |
|---|---|---|---|---|
| G 1/8" (BSPP) | 9.7mm | 28 | Bonded seal washer | Turbo oil drain banjo, sensors |
| G 1/4" (BSPP) | 13.2mm | 19 | Bonded seal / O-ring | EU/JDM oil drain plugs, hydraulic ports |
| G 3/8" (BSPP) | 16.7mm | 19 | Bonded seal / O-ring | Gearbox drain/fill plugs, PS pumps |
| G 1/2" (BSPP) | 20.9mm | 14 | Bonded seal / O-ring | Large block ports, hydraulic manifolds |
Metric Thread Ports (M10 × 1.0, M12 × 1.5…)
Many Japanese, European, and Korean OEM blocks use metric-straight threads for banjo bolt fittings – turbo oil feed, brake calipers, injector line connections, and fuel rail ports. They seal via copper or aluminium crush washers (sealing rings) on each side of the banjo fitting.
| Thread | Pitch | O.D. | Typical Applications | Crush Washer |
|---|---|---|---|---|
| M8 × 1.0 | 1.0mm | 8mm | Small sensor ports, bleeder screws | M8 copper (8mm ID, 14mm OD) |
| M10 × 1.0 | 1.0mm | 10mm | Turbo oil feed banjo (most common) | M10 copper (10mm ID, 16mm OD) |
| M10 × 1.25 | 1.25mm | 10mm | Turbo oil feed (some OEM variants) | M10 copper (10mm ID, 16mm OD) |
| M12 × 1.0 | 1.0mm | 12mm | Brake calipers, larger oil ports | M12 copper (12mm ID, 18mm OD) |
| M12 × 1.25 | 1.25mm | 12mm | Brake calipers, injectors | M12 copper (12mm ID, 18mm OD) |
| M14 × 1.5 | 1.5mm | 14mm | Gearbox banjo, hydraulic clutch | M14 copper (14mm ID, 20mm OD) |
Always replace crush washers on banjo fittings every time you remove them. Aluminium washers can be re-annealed (briefly heat to cherry red, quench) for a single re-use – copper washers should be replaced. Never re-use a compressed/deformed crush washer – the lip profile that creates the seal is destroyed after the first use.
How to Identify Any Thread
Identifying unknown threads is a critical skill. Use this decision flowchart:
Is the thread tapered (OD increases toward the base) or straight?
Tapered: Measure TPI. Count threads per inch.
Straight: Does it have a hex shoulder stop suggesting O-ring compression?
To confirm AN vs BSP on a straight thread: measure thread angle with thread gauge
A thread pitch gauge (one angular, one TPI) is a ~RM30 tool that resolves 95% of identification questions in under 30 seconds. An AN thread gauge set (check if swivel nut threads onto fitting – no side-load wobble = correct standard) works for confirming AN sizes. When in doubt, consult the OEM workshop manual for your engine – it will specify port standards by part number.
Adapter Compatibility Matrix
Common adapter combinations for building AN systems from OEM ports:
| Port / From | AN End | Sealant Required | Rating | Notes |
|---|---|---|---|---|
| 1/8" NPT port | -4 AN male | PTFE tape or anaerobic | Good | Common for oil pressure sensor, gauge ports |
| 1/4" NPT port | -4 or -6 AN male | PTFE tape or anaerobic | Good | Turbo oil feed boss (most JDM engines) |
| 3/8" NPT port | -6 or -8 AN male | PTFE tape or anaerobic | Good | Oil pan drain boss, block port |
| G 1/8" BSPP port | -4 AN | Bonded seal washer | Excellent | Turbo oil feed/drain on many JDM engines |
| G 1/4" BSPP port | -6 AN | Bonded seal washer | Excellent | GTR, RB, SR engines oil ports |
| M10×1.0 banjo | -4 AN banjo end | Copper crush washers | Excellent | Turbo oil feed line entry point |
| -6 ORB port | -6 AN (37° seat adapter) | None – O-ring seals | Excellent | Use ORB-to-AN male adapter with O-ring |
| -12 ORB port (PS pump) | -12 AN | None – O-ring seals | Excellent | Power steering hard line to AN system |
Thread Sealant Guide
PTFE Thread Tape (White)
Use For
NPT and BSPT tapered threads
Not For
AN/JIC flare threads, ORB threads, any fitting with a separate sealing face
Anaerobic Thread Sealant (e.g. Loctite 567)
Use For
NPT, BSPT, and NPSM threads. Good for tapered pipe threads where positive locking is desired.
Not For
AN/JIC 37° flare seats, ORB O-ring counterbores, brake-system fittings
No Sealant
Use For
AN/JIC 37° flare connections, ORB with O-ring, metric banjo with crush washers
Not For
NPT tapered pipe threads (will leak without sealant)
Torque Reference by Thread Standard
| Thread / Standard | Torque (Aluminium Fittings) | Torque (Steel Fittings) | Notes |
|---|---|---|---|
| -3 AN | 3–4 Nm | 4–6 Nm | Small – easy to over-torque. Fingertight + 1/6 turn. |
| -4 AN | 4–6 Nm | 6–9 Nm | Fingertight + 1/6 turn. |
| -6 AN | 10–12 Nm | 13–18 Nm | Most common – mark reference with paint pen after assembly. |
| -8 AN | 16–20 Nm | 20–27 Nm | |
| -10 AN | 24–32 Nm | 36–48 Nm | |
| -12 AN | 40–48 Nm | 55–68 Nm | |
| 1/8" NPT | 7–12 Nm | 14–20 Nm | Approx 3 turns engagement after hand-tight. |
| 1/4" NPT | 12–20 Nm | 20–34 Nm | |
| 3/8" NPT | 20–27 Nm | 34–54 Nm | |
| M10×1.0 banjo | – | 17–23 Nm | With copper crush washers. |
| M12×1.25 banjo | – | 23–30 Nm | With copper crush washers. |
Aluminium AN fittings are significantly softer than steel. Over-torquing aluminium-to-aluminium AN connections is the #1 cause of cracked flare seats and swivel nut stripping. Always torque aluminium fittings to the lower end of the range. A properly assembled and torqued AN connection will not leak – if it leaks, the problem is surface damage, not insufficient torque.
Common Mistakes
Applying PTFE tape to AN/JIC fittings
AN connects via metal-to-metal 37° flare – PTFE tape has no role and prevents proper seating. Apply only to NPT/BSPT tapered threads.
Mistaking G 1/4" BSPP for 1/4" NPT
Both are ~13–14mm OD and similar pitch – they will partially thread together and feel wrong. BSPP is 55° form/19 TPI; NPT is 60° form/18 TPI. Cross-threading these destroys both fittings.
Confusing ORB and AN thread sizes
-6 ORB and -6 AN share UNF 9/16-18 threads and will physically thread together – but ORB seals on an O-ring face, AN on a 37° flare. Using an AN male fitting in an ORB port (or vice versa) will leak.
Over-torquing aluminium fittings
More torque does not fix a leak on AN connections. If a properly torqued joint leaks, the flare seat is damaged. Inspect and replace the damaged component.
Reusing crush washers on banjo fittings
Crush washers seal by permanent deformation. A pre-compressed washer cannot create a reliable seal. Always use new copper or aluminium crush washers – they cost cents.
Browse AN Adapters & Fittings
NPT-to-AN, ORB-to-AN, BSP-to-AN, and metric banjo adapters – all in stock.