Thread Compatibility
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 the skill of measuring thread size and pitch 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.
Figure 2: A brass and PVC internal thread adapter
Selecting ball valves with inch connections for pipes with millimeter connections
Converting between metric thread sizes and standard thread sizes (aka imperial thread sizes or inch thread sizes) is not as straightforward as may be assumed. In the case of selecting a valve with connections designated in inches, it is not appropriate to measure a pipe's diameter with millimeters and then convert directly to inches. Use Table 1 as a guide for what ball valve diameters to choose based on pipe diameter.
Table 1: Conversions between inch and millimeter for ball valve and pipe connections.
| Diameter (in) | DN (mm) | Pipe OD (mm) |
|---|---|---|
| 1/4" | 8 | 13.7 |
| 3/8" | 10 | 17.14 |
| 1/2" | 15 | 21.3 |
| 3/4" | 20 | 26.7 |
| 1" | 25 | 33.4 |
| 1.2" | 32 | 42.2 |
| 1.5" | 40 | 48.3 |
| 2" | 50 | 60.3 |
| 2.5" | 65 | 73 |
| 3" | 80 | 88.9 |
| 4" | 100 | 114.3 |
| 5" | 125 | 141.3 |
| 6" | 150 | 168.3 |
| 8" | 200 | 219.1 |
| 10" | 250 | 273 |
| 12" | 300 | 323.8 |
| 14" | 350 | 355.6 |
| 16" | 400 | 406.4 |
Thread compatibility chart
The following pipe 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 |
Male |
Female NPT, NPTF | thread |
MPT is male NPT, FPT is female NPT, |
|
| 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, |
| 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 |
|
| Female | Male NPSM, NPT, NPTF | ||||
|
National pipe straight hose |
NPSH |
Male | Female NPSH |
washer |
|
| Female | Male NPSH, NPSM, NPT, NPTF | ||||
|
British standard pipe taper |
BSPT |
Male |
Female BSPT | thread |
BSPT threads are known as R threads, |
| Female BSPP | washer | ||||
| Female | Male BSPT | thread | |||
|
British standard pipe parallel |
BSPP |
Male | Female BSPP | washer |
BSPP threads are known as G threads, |
| Female | Male BSPP, BSPT | washer | |||
|
Garden hose thread |
GHT |
Male | Female GHT | washer |
Thread is same for all size hoses, |
| Female | Male GHT | washer | |||
|
Metric parallel |
M |
Male | Female Metric parallel | washer | |
| Female | Male metric parallel, male metric tapered | thread | |||
|
Metric tapered |
- |
Male |
Female metric tapered, MJ | thread | |
| Female metric parallel | washer | ||||
| Female | Male metric tapered | thread | |||
|
Metric threads for high stress |
MJ |
Male | MJ female | washer | |
| 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.
Is NPSM thread compatible with NPT thread?
Female NPSM threads can engage with NPT threads but will not create a pressure seal. Use a sealant, such as Teflon tape, to improve the connection.
