Why Metric and Nominal Threads Should Not Be Forced Together
Aug 9th 2022
Metric and nominal thread systems are not interchangeable just because the parts appear close in size. Thread pitch, thread angle, diameter, and sealing method can all be different.
Forcing two thread standards together can cross-thread the connection, damage the port, and create an immediate leak path.
Why the Threads Do Not Match
Nominal pipe threads such as NPT are built around one thread system. Metric threads are built around another. The pitch and profile may be close enough to start by hand but wrong enough to bind before the fitting seats correctly.
The seal can also be different. Some connections seal through thread interference, some use an O-ring, and some seal against a cone, washer, or bonded seal. Mixing the thread form usually means the sealing surface does not match either.
Use an Adapter Instead of Forcing the Fit
When a machine has mixed standards, the correct solution is a conversion adapter. A proper adapter changes from one thread standard to another while preserving the intended sealing method on each side.
Examples include metric to JIC, NPT to BSPP, and ORB-style adapters where the O-ring boss side needs its own seal and port geometry.
Standardization Helps Maintenance
Keeping a system consistent makes future service easier. Technicians can identify the thread family faster, reduce damaged threads, and avoid replacing ports because an incorrect fitting was forced into place.
Before installation, confirm the thread type, pitch, seal style, and port standard. A fitting that turns a few threads by hand is not proof that the connection is compatible.