Sep 16, 2026Resources

NPT vs BSPP vs BSPT: How to Identify Threads

Learn the practical differences between NPT, BSPP and BSPT threads, including taper, thread angle, sealing method and what to measure before ordering fittings.

NPT vs BSPP vs BSPT: How to Identify the Right Thread

One of the easiest ways to order the wrong fitting is to describe a connection simply as:
“1/2-inch pipe thread.”
That is not enough.
A fitting may use NPT, BSPT or BSPP, and connections with similar nominal sizes can still differ in thread form, pitch, taper and sealing method.
Before ordering a fitting, valve, adapter, quick coupling or hose end, identify the thread correctly.


Quick Comparison

Feature
NPT
BSPT
BSPP
Thread form
Tapered
Tapered
Parallel
Thread angle
60°
55°
55°
Common reference
ASME B1.20.1
ISO 7-1 / related BSP taper standards
ISO 228-1
Typical sealing principle
Thread interference + suitable sealant
Thread interference + suitable sealant
Separate sealing surface/seal
Common notation
NPT
R / Rc, depending on connection
G
The exact connection specification should always be checked against the applicable product drawing or standard.


What Is NPT?

NPT — National Pipe Taper — is a tapered pipe thread commonly encountered in North American industrial equipment.
Its thread form uses a 60° included angle.
Because both mating threads are tapered, tightening creates increasing thread interference.
A suitable thread sealant may be required according to the product, medium, temperature and installation procedure.
NPT should not be identified simply by measuring a nominal outside diameter because pipe-thread nominal size does not equal the measured thread diameter.


What Is BSPT?

BSPT is the tapered form of the British Standard Pipe thread family.
Like NPT, it is tapered, but its thread form uses a 55° included angle, not NPT's 60°.
Pitch can also differ.
That means an NPT and BSPT connection may sometimes appear to start threading together even though they are not the same thread.
“It screws in” is not a valid thread-identification method.
Forcing mismatched threads can damage the connection and does not establish a reliable pressure seal.


What Is BSPP?

BSPP is the parallel form of the British Standard Pipe thread family.
Unlike NPT and BSPT, the thread itself is not tapered.
A BSPP connection therefore normally depends on a defined sealing arrangement such as a bonded seal, gasket, O-ring or mating sealing surface, depending on the connection design.
This distinction is critical:
The thread can provide mechanical engagement while another feature provides the pressure seal.
That is why identifying only the thread diameter is not enough.
Parker's instrumentation fitting guide makes the same fundamental distinction: BSPP is parallel and uses a peripheral sealing arrangement, while BSPT and NPT are tapered thread forms.


Why NPT and BSP Threads Are Commonly Confused

Three things cause most identification errors.

1. Similar nominal sizes

A nominal 1/4, 3/8 or 1/2 designation does not mean that different thread standards share identical geometry.

2. Similar appearance

Without a thread gauge or careful measurement, tapered pipe threads can look very similar.

3. Partial engagement

Two incorrect thread forms may sometimes engage for a few turns.
That does not make them compatible.


How to Identify an Unknown Thread

If you have an existing component but no reliable part number, check the connection systematically.

Step 1 — Determine whether it is tapered or parallel

Measure the thread diameter at more than one point along the thread.
A tapered thread changes diameter along its length.
A parallel thread remains substantially constant.


Step 2 — Measure the thread diameter

Use a caliper to measure the male thread major diameter or the corresponding female-thread dimension.
Do not immediately convert the measured diameter into a nominal pipe size.
Use it as one piece of evidence.


Step 3 — Determine the pitch

Use a thread-pitch gauge where possible.
For inch-based pipe threads, pitch is commonly expressed as threads per inch (TPI).
Two threads with similar diameters can have different pitches.


Step 4 — Identify the thread form

NPT and BSP threads use different flank angles:
NPT: 60°
BSP: 55°
Diameter + pitch + taper/parallel form together provide much stronger identification than any one measurement alone.


Step 5 — Inspect the sealing method

Look beyond the threads.
Does the connection use:
  • tapered thread interference;
  • an O-ring;
  • a bonded washer;
  • a gasket;
  • a cone/seat;
  • another metal-to-metal sealing surface?
The sealing geometry can help identify the connection family and is essential when selecting an adapter.


Can NPT and BSP Be Connected Together?

They should not be treated as directly interchangeable.
Even when two connections appear to engage, differences in thread angle, pitch and geometry can prevent correct engagement and sealing.
If an NPT system must connect to a BSP system, use an adapter specifically designed for the two connection standards.
Do not create the conversion by forcing unlike threads together.


What Should You Send With an RFQ?

If you are unsure of an existing thread, send:
  • clear side-view photographs;
  • male or female connection;
  • measured thread diameter;
  • measured pitch/TPI;
  • whether the thread appears tapered or parallel;
  • sealing-face photographs;
  • product type;
  • operating pressure;
  • operating temperature;
  • process medium;
  • material;
  • quantity.
If available, also send the original:
manufacturer + complete part number + drawing.
That is usually more reliable than trying to identify a connection from a photograph alone.


One More Point: Thread Type Is Not the Whole Connection

This becomes especially important with hydraulic fittings.
Terms such as JIC, ORFS and DIN describe connection systems with their own sealing geometry. Identifying the thread alone does not necessarily identify the complete connection.
For example, the thread may provide mechanical retention while a flare, O-ring face or cone provides the actual seal.
So when replacing an unknown connection, identify:
Thread + sealing surface + size + connection standard
—not just the thread.


Need Help Identifying a Connection?

StarkValve supplies threaded connections across:
  • instrumentation tube fittings;
  • hydraulic fittings and adapters;
  • instrumentation valves and manifolds;
  • quick couplings;
  • hose and flexible connections;
  • pipe fittings.
Send us the existing part number, photos, dimensions and operating conditions.
We can review the connection before quotation.
CTA: Identify Your Connection