Sensor fittings and threads: identifying metric, BSP, NPT and inch threads

Sensor threads, fittings and line connections on vehicles and machinery come from different standard systems. Metric threads, British pipe threads and American taper threads are in some cases so close in diameter and pitch that the wrong part can be screwed in several turns. The result is leaking joints and damaged threads.

The following sections sort the common systems by dimensions, thread angle and sealing principle, and point out the combinations that are frequently mixed up.

A thread is fully defined by seven characteristics:

1. Nominal diameter
   For fastening threads: the theoretical major diameter (metric or inch)
   for pipe threads: the inch figure is purely a size designation and has no direct relation to the outside or inside diameter.

2. Pitch
   metric: in mm → M10×1 = 1 mm axial advance per turn
   inch: in threads per inch → 1/4-20 UNC = 20 threads per inch (25.4 mm)

3. Thread angle
   60° for metric ISO threads (M, MF), UNC/UNF/UNEF, NPT and NPTF
   55° for Whitworth, BSW, BSF, BSP, G and R
   Special forms: 30° for trapezoidal and round threads, 47.5° for BA (British Association), 80° for Pg (steel conduit thread)

4. Profile (the shape of the thread form)
   V-thread (metric ISO, UNC/UNF, NPT)
   Whitworth profile with rounded crests and roots (BSW, BSF, BSP, G, R)
   Trapezoidal thread (Tr, DIN 103)
   Buttress thread (DIN 513)
   Round thread (DIN 405)
   Steel conduit thread (Pg, DIN 40430)

5. Form
   parallel (M, G, UNF): never seals in the thread but on a sealing face outside it (shoulder, sealing washer, O-ring), or not at all (pure fastening threads)
   tapered 1:16 (R, Rc, NPT, NPTF, M10×1 keg. to DIN 158): seals in the thread only

6. Hand
   right-hand or left-hand (suffix LH)

7. Number of starts
   single- or multi-start

Metric threads (ISO 261 / ISO 724 / DIN 13)

Coarse thread: one standard pitch per nominal diameter (M5 = 0.8; M6 = 1.0; M8 = 1.25). Fine thread: smaller pitch, always stated in the designation (M10×1). Sensor and engine threads are often fine threads, which hold better with a short thread engagement in thin-walled aluminium. Metric taper threads (DIN 158) exist but are rare on sensors. The standard is a parallel fine thread sealed at a shoulder or sealing washer.

Typical applications: M5/M6/M8 = small sensors, blanking plugs · M10×1 = de facto standard for oil pressure and oil temperature sensors (VDO, Bosch) · M10×1.25 = e.g. Suzuki DR-Z4S temperature sensor · M12×1.5 / M14×1.5 = oil drain and blanking plugs (KTM LC8 oil filter bolt M14×1.5) · M16×1.5 / M18×1.5 = larger plug and sensor ports, timing chain tensioners · M18×1.5 = standard for lambda sensors (wideband; stock narrowband sensors on motorcycles are mostly M12×1.25).

• BSPP / G (ISO 228-1): parallel, 55°, does not seal in the thread → sealing washer, O-ring or shoulder required.
• BSPT / R (ISO 7-1 / EN 10226): tapered 1:16, 55°, seals in the thread (sealant recommended). R = external taper thread.
• NPT (ASME B1.20.1): tapered 1:16, 60°, sealant required.
• NPTF / Dryseal (ASME B1.20.3): can seal dry; the crests deform.
• NPSM: parallel, 60°.

Taper 1:16 means the diameter changes by 1 mm per 16 mm of thread length.

Note: “1/8 inch” is the old nominal pipe size, not the outside diameter of the thread. A 1/8" thread measures roughly 9.7–10.3 mm across, not 3.175 mm. The NPT figures are nominal outside diameters; on a taper, the measured diameter depends on where you measure.

PT (JIS B0203) = BSPT = ISO 7 R, tapered, seals in the thread. PF (JIS B0202) = BSPP = ISO 228 G, parallel. PS = parallel internal thread. Interchangeable with BSP of the same size. Caution with line connections: Japanese flare fittings use a 30° sealing cone (JIS B8363) and do not fit a 37° AN/JIC seat. In practice, many Japanese oil pressure switches have a 1/8" PT thread – the most common reason for “almost fits”.

NPT dominates on US vehicles and Harley-Davidson (oil pressure switches mostly 1/8-27 NPT, some 1/4-18 NPT). UNF threads on senders: 7/16-20, 1/2-20, 9/16-18 UNF.

• AN/JIC (SAE J514): 37° flare, straight UNF thread, metal-to-metal seal without sealant. Dash size = tube outside Ø in 1/16". AN-4 = 7/16-20 UNF, AN-6 = 9/16-18 UNF.
• SAE ORB (J1926): same UNF threads as JIC, O-ring seal, cannot be combined with JIC (the thread engages, the seal fails).

Sealing in the thread – tapered threads only (NPT, NPTF, R, Rc)

Sealing method:
Mechanical interference → flanks deform, NPTF (Dryseal) only
PTFE tape → cheap, electrically insulating
Anaerobic thread sealant → vibration-resistant
Hemp/sealing cord → used in plumbing.

Sealing at the shoulder/face – parallel threads (M, G, UNF, BSPP)

Sealing elements:
Copper/aluminium sealing washer (DIN 7603)
Paper/fibre/rubber sealing washer
Bonded seal / Usit ring (rubber-metal composite washer)
O-ring
Sealing/cutting edge form B or E (DIN 3852 / ISO 1179)

Single-wire sensors and switches take GND through the thread and housing. PTFE tape or insulating sealant breaks the GND path → no signal or jumping readings. Solutions: use conductive sealant sparingly and measure the contact resistance, a separate GND wire or two-wire, isolated-GND sensors. Stainless steel in aluminium: galvanically uncritical in an oil-wetted installation.

Tools: pitch/thread gauge (60° metric vs. 55° Whitworth), calliper (Ø at the start and end → detect taper), carefully screw in a known mating part by hand.

• A2 (1.4301): resistant to oil, coolant, fuel and heat.
• A4 (1.4401/1.4404): better against salt and winter conditions, slightly more expensive.
• Brass: conductive, inexpensive, less strong – less suitable for locations exposed to vibration.
• Zinc-plated steel: inexpensive, rusts at cut edges.
• Aluminium: light, threads easily damaged.

An oil pressure sensor does not have to sit where the pressure is tapped. As soon as the mounting location is cramped, hot or exposed to heavy vibration, the sensor moves to a pressure line. In motorsport, the AN system has become the established solution for this.

AN stands for Army-Navy and comes from US military standardisation. The civilian equivalent is JIC, named after the Joint Industry Council and standardised in SAE J514 and ISO 8434-2. Both describe the same type of connection: a male cone with a 37° flare angle presses against a matching female cone, metal to metal.

What matters in practice is the division of labour at this joint. The UNF thread provides the clamping force; the seal is made at the cone. That is why neither PTFE tape nor sealing paste belongs on an AN connection. Sealant goes on the other end of the thread adapter, where a tapered NPT or BSPT thread goes into the engine.

The number after the dash is the tube outside diameter in sixteenths of an inch. AN-4 therefore means 4/16 inch, i.e. 1/4 inch or 6.35 mm. Each size has its own thread (UNF, from AN-12 upwards UN).

On motorcycles you are almost always working between AN-3 and AN-10. The usable inside diameter is well below the tube size and depends on the hose type. For AN-4 it is roughly 4.8 to 5.5 mm depending on construction.

Three types are common, and they cannot be mixed. Each hose needs its matching fitting type.

• Rubber or textile hose with stainless steel braid: synthetic core of NBR or CPE, stainless steel outer braid. For oil, fuel and air. Depending on size, working pressures are in the range of a few tens of bar. Not for brakes.
• PTFE hose with stainless steel braid: PTFE liner, temperature-resistant from about −70 to +250 °C, impermeable and therefore the first choice for brake fluid, E85 and hot oil.
• Push-Lock: the rubber hose is pushed onto a barbed fitting without tools. Only suitable for low pressures and cannot be combined with swivel fittings.

Cone shapes look similar but are not interchangeable:

• SAE 45° flare: partly the same thread as AN, different sealing angle. The joint can be screwed together but will not seal.
• SAE ORB: the same UNF thread as JIC, but the seal is an O-ring on the stud end, not a cone. Not interchangeable.
• JIS 30° flare: common on Japanese vehicles, does not fit a 37° seat.

If there is not enough room for the sensor at the mounting point, a short pressure line can help: the pressure is tapped at the engine, and the sensor sits decoupled in a protected location, e.g. on the frame or on a bracket.

What does 1/8 inch mean on a pipe thread?

Not 3.175 mm. On pipe threads, the inch figure is a historical size designation, not a dimension. A G 1/8 measures 9.73 mm across the outside, a 1/8" NPT 10.29 mm. Only on fastening threads such as 1/4" BSW does the inch figure correspond to the outside diameter.

Do NPT and BSP fit together?

No. NPT has a 60° thread angle, BSP 55°. They can be screwed into each other for a few turns, but then bear only on the thread crests. The result leaks, and both threads are damaged.

How can I tell whether a thread is tapered?

Measure the outside diameter at the start and at the end of the thread. If the diameter decreases towards the tip, it is tapered, i.e. NPT, NPTF, R or Rc. If it stays the same, it is parallel, i.e. M, G or UNF.

Which sealant belongs on a sensor thread?

Tapered threads seal in the thread and need sealant; an anaerobic thread sealant is the usual choice. Parallel threads seal via a sealing washer, O-ring or shoulder and need no sealant in the thread. Never use PTFE tape on single-wire sensors, because it interrupts the GND contact.

What does AN-4 or Dash 4 stand for?

For the tube outside diameter in sixteenths of an inch. AN-4 equals 4/16 inch, i.e. 1/4 inch or 6.35 mm, and has a 7/16-20 UNF thread with a 37° sealing cone.

How do I tell 1/8" BSP from 1/8" NPT?

With a calliper and a pitch gauge. BSP has 28 threads per inch at 9.73 mm outside diameter, NPT 27 threads at 10.29 mm. If you start the wrong part, it goes in about two turns and then binds. Do not force it any further at this point, or both threads will be ruined.

Are PT and PF the same as BSPT and BSPP?

Yes, they are just the Japanese designations. PT to JIS B0203 corresponds to BSPT, PF to JIS B0202 corresponds to BSPP. In practice this matters because many Japanese oil pressure switches have 1/8" PT, i.e. BSPT, not NPT.

• Bosch PST-F1 datasheet 70496907
• Bosch PST-F1 datasheet Rev. 51
• Bosch PSS-10 datasheet
• Hydraulic Insight – BSP Thread Size Chart
• Hydraulic Insight – NPT Thread Size Chart
• Voelkel – G, R, Rp, Rc & NPT
• Chillelec – G1/8 NPT vs BSPP
• Wikipedia – British Standard Pipe
• Wikipedia – National pipe thread
• anzugsmoment.de – BSP thread (German)
• anzugsmoment.de – NPT thread (German)
• Jiayuan – BSPP/BSPT/PF/PT/G/R
• Hoseflex – JIS B0203
• Union Hydraulics – JIC
• GageRef – SAE ORB
• Henkel – Loctite 542
• AIMS – Loctite 577 / 567
• Fullerkreg – Galling
• Fastenerdata – Galling
• Creative Motorsport – Bosch PSS / installation note