Thread Compatibility

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.

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.

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	NPT	Male	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	NPTF	Male	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	NH/NST	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	NPSM	Male	Female NPSM, NPSH	Mechanical or washer	ASME B1.20.1
		Female	Male NPSM, NPT, NPTF
National pipe straight hose	NPSH	Male	Female NPSH	washer	ASME B1.20.1
		Female	Male NPSH, NPSM, NPT, NPTF
British standard pipe taper	BSPT	Male	Female BSPT	thread	BSPT threads are known as R threads, ISO 7
			Female BSPP	washer
		Female	Male BSPT	thread
British standard pipe parallel	BSPP	Male	Female BSPP	washer	BSPP threads are known as G threads, ISO 7
		Female	Male BSPP, BSPT	washer
Garden hose thread	GHT	Male	Female GHT	washer	Thread is same for all size hoses, ASME B1.20.7
		Female	Male GHT	washer
Metric parallel	M	Male	Female Metric parallel	washer	ISO 724
		Female	Male metric parallel, male metric tapered	thread
Metric tapered	-	Male	Female metric tapered, MJ	thread	ISO 724
			Female metric parallel	washer
		Female	Male metric tapered	thread
Metric threads for high stress	MJ	Male	MJ female	washer	ISO 5855
		Female	MJ male, M male	washer
Unified Thread Standard	UTS	Male	Female UTS	washer	ASME B1.1

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

FAQs

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.