Thread Compatibility

Thread Compatibility

Tee-joint pipe connection with threaded connection points

Figure 1: Tee-joint pipe connection with threaded connection points

Thread compatibility ensures that an external thread can properly connect with an internal thread of the same standard. Compatible threads have the same geometry, which can be difficult to notice with the naked eye. Learning how to measure the size and pitch of threads can help ensure compatibility. This article describes the compatibility of the most common thread types in Europe, Canada, and the United States.

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Incompatible threads

Despite some claims, using incompatible threads is, at best inefficient and, at worst life-threatening. Some claim that pipe sealant or thread tape can prevent leaks with incompatible threads. This is not true. High pressure can disconnect incompatible threads. One of the components, such as a pillar valve on an air cylinder, can fly off the system and potentially injure someone near the point of failure. Media leakage can also occur, which is dangerous if the media is toxic. Read our thread design article for more details on each thread design parameter.

Thread adapters

Compatible threads are likely available when looking for threaded fasteners (e.g., bolts and nuts) to use with a locally-purchased appliance. However, when purchasing an appliance from another part of the world, the issue of incompatible threads can arise. For example, if someone from the United States buys an espresso machine from Italy. The espresso machine will likely have a British Standard Pipe (BSP) threaded connection point. However, the person who purchased the device will probably only have Unified Thread Standard threaded fasteners on hand.

Consider a thread adapter rather than risking failure, damage, or injury that can occur with incompatible threads. Thread adapters, such as the one seen in Figure 2, solve the issue of incompatible threads. Adapters come in many different materials and are applicable for joining threads of several standards.

A brass and PVC internal thread adapter

Figure 2: A brass and PVC internal thread adapter

Thread compatibility chart

The following thread compatibility chart details which thread types are suitable for connecting. Straight thread connections require a washer, whereas tapered connections do not. With straight threads, the external and internal threads are parallel with each other. Therefore, the crests, roots, and flanks do not interfere with one another. This leaves a small gap that media can flow through and cause leakage. Tapered connections do not have this issue, however thread sealant or Teflon tape is still necessary.

Name Abbreviation Male/Female Compatible with Seal Notes

National pipe taper



Female NPT, NPTF thread

MPT is male NPT, FPT is female NPT,

ASME B1.20.1

Female NPSM, NPSH washer
Female Male NPT, NPTF thread

National pipe tapered fuel



Female NPTF, NPT thread

NPTF threads are known as dryseal threads,

ASME B1.20.1

Female NPSH, NPSM washer
Female Male NPTF, NPT thread

National hose/National standard thread


Male Female NH/NST Gasket in female coupling

NH and NST thread are the same. Most common thread type used by U.S. fire departments

Female Male NH/NST Square face on male coupling

National pipe straight mechanical


Male Female NPSM, NPSH

Mechanical or washer

ASME B1.20.1

Female Male NPSM, NPT, NPTF

National pipe straight hose


Male Female NPSH


ASME B1.20.1


British standard pipe taper



Female BSPT thread

BSPT threads are known as R threads,


Female BSPP washer
Female Male BSPT thread

British standard pipe parallel


Male Female BSPP washer

BSPP threads are known as G threads,


Female Male BSPP, BSPT washer

Garden hose thread


Male Female GHT washer

Thread is same for all size hoses,

ASME B1.20.7

Female Male GHT washer

Metric parallel


Male Female Metric parallel washer

ISO 724

Female Male metric parallel, male metric tapered thread

Metric tapered



Female metric tapered, MJ thread

ISO 724

Female metric parallel washer
Female Male metric tapered thread

Metric threads for high stress


Male MJ female washer

ISO 5855

Female MJ male, M male washer

Unified Thread Standard


Male Female UTS



Read our thread tolerance article for more details on thread tolerance classes.


Is G thread compatible with NPT?

G thread, or British Standard Pipe Parallel thread, is incompatible with National Pipe Taper (NPT) thread. If necessary, use a thread adapter to join these two thread types.

What is fire hose thread compatible with?

Fire hoses around the world have many different thread standards. If necessary, look up what thread types are compatible with fire hoses in the local region.

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