NPT Engagement and Torque
"Wrench tight plus a little more"
Tapered pipe threads (NPT) do not have a specified torque rating. Traditionally, they are tightened by technicians until they seal. That typically means wrench tight, plus a little more.
This ambiguous guideline is a frustrating if you are developing a procedure for the mass production of NPT connections. You would like assurance that all of the threaded connections will seal without having to proof test them individually.
The best method of assuring that your threads will function as intended is to do the following:
NPT Tightening Procedure
Ensure the threads are clean and free from burs
Verify that the female thread is cut properly with “Go” / “No Go” gauge
Verify that the male thread is milled properly with a “Go” / “No Go” gauge ring
If both threads are cut properly, tighten them by hand until you cannot turn it any further - This is the hand tight engagement point
Use a wrench to tighten the threaded joint further. For NPT fittings smaller than 1", this is typically about 2 full turns. You can refer to the Machinery's Handbook for the exact wrench tight distance.
NPT in Aluminum and Other Soft Materials
NPT joints can be successfully used in Aluminum, but they can also pose serious problems such as galling.
Because they seal by interference, the threads can be easily damaged if the connection is assembled and disassembled regularly.
If this is the case, it might make sense to look at other thread types which are less prone to damage with repeat use.
Note: Be careful with using Aluminum male threads with Aluminum female threads as they are prone to seizing and galling.
NPT Common Mistakes
A few common mistakes with NPT connections is to:
Use thread tape
Use thread sealant
Use thread lubricant
NPT connections are designed to create a leak tight mechanical connection without the assistance of any of the above products.
Further Reading
For further reading, you can refer to the "Machinery's Handbook".