NPT vs NPTF vs BSPP vs BSPT Thread Gauges: Buyer’s Selection Guide

Choosing between NPT, NPTF, BSPP and BSPT thread gauges is not simply a matter of matching the nominal pipe size. Buyers must confirm the governing standard, thread profile, taper or parallel form, internal or external thread, sealing method and required gauging system.

Need gauges for this? GageTools makes NPT, NPTF, BSPP and BSPT thread gauges with MOQ 1 piece: plug gauges from USD 20, ring gauges from USD 30, and standard stock sizes ship within 72 hours. Send your thread size for a quote.

An incorrect pipe thread gauge may appear to fit the component while still failing to verify the thread specified on the engineering drawing. This guide compares NPT vs BSP thread gauges, explains NPT vs NPTF and BSPP vs BSPT, and provides a practical RFQ checklist for industrial buyers.

For an ISO-focused explanation of parallel and taper BSP systems, see our BSP thread gauge guide for BSPP, BSPT, G and R threads.

Quick Answer: Which Pipe Thread Standard Do You Need?

• NPT: American 60-degree taper pipe thread under ASME B1.20.1. It is commonly inspected with an NPT L1 plug or ring gauge.
• NPTF: American Dryseal pipe thread under ASME B1.20.3, with gauging methods covered by ASME B1.20.5. Its crest, root and inspection requirements differ from NPT.
• BSPP: Common trade name for the parallel G thread system under ISO 228-1. The pressure seal is not made by the thread itself.
• BSPT: Common trade term for ISO 7 pipe threads. The formal designations are R for taper external threads, Rc for taper internal threads and Rp for parallel internal threads.

Procurement rule: Use BSPP or BSPT as search terms, but place the order using the exact standard and formal designation shown on the drawing.

NPT vs NPTF vs BSPP vs BSPT Comparison

Thread system	Standard	Profile	Parallel or taper	Typical designation	Sealing concept
NPT	ASME B1.20.1	60°	1:16 taper on diameter	1/4-18 NPT	Pressure applications normally follow the specified assembly and sealing-compound requirements
NPTF	ASME B1.20.3	60°	1:16 taper for taper NPTF	1/4-18 NPTF-1 or NPTF-2	Dryseal form designed to control spiral leakage without requiring sealing compound when correctly manufactured and assembled
BSPP / G	ISO 228-1	55°	Parallel internal and external	G 1/2 or G 1/2 A	Separate sealing face, washer, gasket or O-ring is normally required
BSPT / ISO 7	ISO 7-1	55°	R and Rc are taper; Rp is parallel	R 1/2, Rc 1/2 or Rp 1/2	Pressure-tight joint is formed through the mating thread system with the appropriate jointing medium

The table is a selection summary, not a substitute for the applicable standard. Final gauge acceptance limits must be based on the drawing, specified standard edition and buyer-approved inspection plan.

Why Pipe Threads With the Same Size May Not Be Interchangeable

Nominal pipe size is a designation rather than the measured outside diameter. Two threads can carry the same nominal size—or even the same threads per inch—and still use different profiles, pitch diameters, taper definitions and inspection rules.

Nominal size	BSP threads per inch	NPT threads per inch	Buyer warning
1/8	28	27	Different pitch and thread profile
1/4	19	18	Do not select by nominal size alone
3/8	19	18	Confirm BSP or NPT on the drawing
1/2	14	14	Same TPI but different 55° and 60° profiles
3/4	14	14	Same TPI does not establish compatibility
1	11	11.5	Different pitch, profile and gauging system

Use the BSP and NPT thread size chart for initial reference, but use the complete engineering callout when ordering production gauges.

NPT Thread Gauges Under ASME B1.20.1

NPT is a general-purpose American taper pipe thread with a 60-degree included angle and a 1:16 taper on diameter. It is widely used for valves, pipe fittings, pneumatic equipment, hydraulic components and industrial fluid systems.

NPT Internal Thread Inspection

An internal NPT thread is commonly inspected with an NPT L1 taper thread plug gauge. Acceptance is evaluated by the position of the workpiece face relative to the gauge reference notch or step at hand-tight engagement.

NPT External Thread Inspection

An external NPT thread is commonly inspected with an NPT L1 taper thread ring gauge. The axial position of the pipe or fitting relative to the ring-gauge reference face is evaluated according to ASME B1.20.1.

NPT taper gauging should not be described as the ordinary straight-thread GO/NO-GO “two-turn rule.” The correct result depends on the standard-defined reference position, standoff and permitted variation.

NPTF Dryseal Gauges Are Not the Same as NPT Gauges

NPTF threads are based on the American pipe-thread system but apply modified crest and root truncation requirements. When conforming mating threads are assembled, controlled crest-and-root contact helps interrupt the spiral leakage path.

The product thread requirements are defined by ASME B1.20.3, while practical NPTF inspection methods and gauges are covered by ASME B1.20.5.

NPTF workpiece	Common coordinated gauges	Purpose
External taper thread	L1 and L2 ring gauges	Coordinate the applicable external-thread inspection requirements
Internal taper thread	L1 and L3 plug gauges	Coordinate the applicable internal-thread inspection requirements
NPTF Class 2 inspection	Crest/root inspection or direct measurement as specified	Verify additional crest and root truncation requirements

NPTF-1 and NPTF-2 refer to different inspection requirements, not simply “loose” and “tight” tolerance grades. The complete designation and required inspection method must therefore appear in the RFQ.

BSPP Thread Gauges: ISO 228-1 G Threads

BSPP is the common commercial name for the G parallel pipe-thread system defined by ISO 228-1. Both the internal and external threads are parallel and use a 55-degree thread profile.

ISO 228-1 states that these threads are intended for mechanical assembly and are not intended to make a pressure-tight joint on the thread. Where pressure sealing is required, the connection normally uses sealing surfaces outside the thread together with a washer, gasket, O-ring or another specified seal.

• Internal G thread: Specify G plus the nominal size, such as G 1/2.
• External G thread: Include external tolerance class A or B when required, such as G 1/2 A.
• Left-hand thread: Add LH when the thread is not right-hand.
• Internal-thread inspection: Use the applicable ISO 228-2 plug-gauge system.
• External-thread inspection: Use the applicable ISO 228-2 ring-gauge system.

ISO 228-2 may require several GO and NOT GO gauges because different elements of the cylindrical 55-degree thread profile must be verified. Do not assume that one generic plug or ring is the complete inspection system.

BSPT Thread Gauges: ISO 7 R, Rc and Rp Threads

BSPT is widely used as a search and trade term, but the more accurate purchasing callouts are the ISO 7-1 symbols R, Rc and Rp.

Designation	Workpiece thread	Thread form	Typical mating arrangement
R	External thread	Taper, 1:16 on diameter	R external with Rp or Rc internal
Rc	Internal thread	Taper, 1:16 on diameter	Rc internal with R external
Rp	Internal thread	Parallel	Rp internal with R external

This distinction matters because “BSPT” does not always mean that both mating threads are tapered. Rp is a parallel internal thread that belongs to the ISO 7 pressure-tight joint system.

ISO 7-2 specifies limit-gauge verification methods for taper external threads, taper internal threads and parallel internal threads covered by ISO 7-1. The exact designation must therefore be confirmed before selecting the plug or ring gauge.

G and Rp Are Both Parallel—But They Are Not the Same Purchasing Callout

G and Rp threads are both parallel and use related 55-degree pipe-thread geometry, but they belong to different joint and gauging systems:

• G: ISO 228-1 mechanical fastening thread; sealing is arranged outside the thread.
• Rp: ISO 7-1 parallel internal thread intended to mate with an R taper external thread as part of a pressure-tight threaded joint.

Do not replace an Rp specification with G—or combine components from the two systems—without engineering approval. Apparent assembly does not prove conformity or leak-tight performance.

Which Pipe Thread Gauge Should You Order?

Workpiece callout	Thread location	Gauge direction	Standard to confirm
NPT	Internal	NPT L1 taper plug gauge; additional checks if specified	ASME B1.20.1
NPT	External	NPT L1 taper ring gauge; additional checks if specified	ASME B1.20.1
NPTF	Internal	Coordinated L1/L3 plug gauges and additional controls as specified	ASME B1.20.3 and B1.20.5
NPTF	External	Coordinated L1/L2 ring gauges and additional controls as specified	ASME B1.20.3 and B1.20.5
G / BSPP	Internal	Applicable parallel thread plug gauges	ISO 228-1 and ISO 228-2
G / BSPP	External	Applicable parallel thread ring gauges	ISO 228-1 and ISO 228-2
Rp	Internal	Parallel internal pipe-thread plug-gauge system	ISO 7-1 and ISO 7-2
Rc	Internal	Taper internal pipe-thread plug-gauge system	ISO 7-1 and ISO 7-2
R	External	Taper external pipe-thread ring-gauge system	ISO 7-1 and ISO 7-2

View available pipe thread gauges, thread plug gauges and thread ring gauges. Special specifications can be produced through the custom thread gauge service.

A Thread Gauge Pass Does Not Automatically Prove Leak-Tightness

A thread gauge verifies the thread elements controlled by its applicable standard and gauge member. Leak performance can also depend on:

• Mating component design and material
• Thread damage, burrs and surface condition
• Plating, coating or surface treatment
• Specified jointing medium, sealant, washer or O-ring
• Assembly engagement and application-specific torque
• Temperature, pressure and operating fluid

Pressure-critical assemblies should use the required functional gauge inspection together with the specified pressure or leak test. ISO 7-2 also notes that limit gauging alone does not completely control every thread-quality characteristic.

Seven Steps for Selecting the Correct Pipe Thread Gauge

1. Read the Complete Drawing Callout

Look for NPT, NPTF-1, NPTF-2, G, R, Rc or Rp—not only the nominal size. Include the specified standard edition if the customer contract requires one.

2. Confirm Internal or External Thread

Internal workpiece threads normally require plug gauges. External workpiece threads normally require ring gauges.

3. Confirm Parallel or Taper Form

Never determine this from a short visual inspection. Confirm it from the thread symbol, drawing or approved part specification.

4. Specify the Complete Size

Include the nominal pipe size and pitch or TPI where applicable. Do not order from a caliper measurement of the component diameter alone.

5. Define the Required Gauge Members

State whether you need an L1, L2, L3, coordinated set, crest/root check gauge, ISO limit-gauge set, working gauge, master gauge or another buyer-approved configuration.

6. Confirm the Workpiece Surface Condition

Tell the manufacturer whether the part is inspected before or after plating, coating or another surface treatment. The approved gauge limits or allowance may depend on this condition.

7. Define Documentation Before Quotation

Request the required calibration certificate, measured-value report, serial-number traceability, material record and buyer-specific marking before the order is placed.

Common Pipe Thread Gauge Ordering Mistakes

• Writing only “1/2-inch pipe gauge” without specifying NPT, NPTF, G, R, Rc or Rp
• Assuming BSPP and BSPT use the same gauge because both are called BSP
• Assuming Rp is a taper internal thread
• Using the same gauge to certify NPT and NPTF products
• Selecting a thread from nominal size or TPI alone
• Failing to state whether the workpiece thread is internal or external
• Ordering only one NPTF gauge member when the inspection plan requires coordinated gauges
• Omitting G external tolerance class A or B
• Failing to specify right-hand or left-hand thread
• Omitting pre-plating or post-plating inspection condition
• Requesting a “certificate” without defining measured values or traceability requirements

Pipe Thread Gauge RFQ Checklist

Copy the following template into your purchasing email. A complete RFQ reduces specification questions and helps the manufacturer provide a more accurate price and lead time.

Pipe Thread Gauge RFQ

1. Workpiece thread callout:
   Example: 1/4-18 NPT
   Example: 1/4-18 NPTF-2
   Example: G 1/2 A
   Example: R 1/2, Rc 1/2 or Rp 1/2

2. Governing standard and required edition:

3. Workpiece thread:
   Internal / External

4. Thread direction:
   Right-hand / Left-hand

5. Gauge requirement:
   Plug / Ring / Complete set
   L1 / L2 / L3 / Crest-root check / ISO limit-gauge set
   Working gauge / Master gauge

6. Workpiece material:

7. Inspection condition:
   Before plating / After plating / Uncoated

8. Quantity:

9. Required documents:
   Inspection report
   Calibration certificate
   Actual measured values
   Traceability requirement
   Material certificate

10. Required marking:
    Buyer part number / Serial number / Logo / Other

11. Shipping country:

12. Required delivery date:

13. Drawing or specification attached:
    Yes / No

Calibration and Inspection Documents to Request

For supplier approval, incoming inspection and controlled production, buyers commonly request:

• Gauge identification and serial number
• Thread standard, designation and gauge member
• Inspection date and conformance result
• Actual measured values where required
• Reference-master or measurement-equipment information
• Calibration traceability route
• Recommended recalibration information when requested

Review available calibration and inspection documentation before finalizing the order.

Standards Referenced in This Guide

• ASME B1.20.1 — Pipe Threads, General Purpose, Inch
• ASME B1.20.3 — Dryseal Pipe Threads, Inch
• ASME B1.20.5 — Gaging for Dryseal Pipe Threads
• ISO 228-1 — Parallel Pipe Thread Dimensions, Tolerances and Designation
• ISO 228-2 — Verification of ISO 228 Threads by Limit Gauges
• ISO 7-1 — Pipe Threads for Pressure-Tight Joints
• ISO 7-2 — Verification of ISO 7 Threads by Limit Gauges

Always confirm the contractually required edition and the latest applicable requirements before manufacturing or accepting inspection gauges.

---

Frequently Asked Questions

Can an NPT gauge be used to inspect an NPTF thread?

Not as the complete NPTF acceptance method. NPTF has additional crest, root and coordinated gauging requirements. Use the exact NPTF designation and the applicable ASME B1.20.5 inspection system.

Are BSPP and BSPT the same?

No. BSPP generally refers to parallel G threads under ISO 228-1. BSPT is a trade term associated with ISO 7 threads, formally designated R, Rc or Rp.

Are NPT and BSP interchangeable when the TPI is the same?

No. Matching TPI does not establish compatibility. NPT uses a 60-degree profile, while BSP systems use a 55-degree profile. Diameters, taper definitions, designations and gauging requirements also differ.

Do I need a plug gauge or ring gauge?

Internal workpiece threads normally require plug gauges. External workpiece threads normally require ring gauges. The gauge must also match the exact pipe-thread system and required gauge member.

What do R, Rc and Rp mean?

Under ISO 7-1, R identifies a taper external thread, Rc identifies a taper internal thread and Rp identifies a parallel internal thread.

Do I need L1, L2 and L3 gauges?

It depends on the standard, thread designation and buyer inspection plan. NPTF inspection commonly coordinates L1 and L2 gauges for external threads and L1 and L3 gauges for internal threads. Send the drawing before ordering.

Does passing a pipe thread gauge prove that the joint will not leak?

No. Gauge acceptance verifies the thread elements controlled by the specified gauge. Joint design, sealing method, surface condition, assembly and pressure testing can also affect leak performance.

Can calibration documents be supplied with pipe thread gauges?

Yes. Inspection reports, calibration certificates, measured values and serial-number traceability can be supplied when defined in the RFQ.

More Specification Points to Confirm

• Check Gauge Wear and Notch Alignment Standards: Because taper pipe gauges seat on a conical plane, minor surface wear causes significant depth shifting.