Plant & Works Engineering
End terminations identification – part 3
Published:  04 August, 2016


Main characteristics and geometry

Unified thread form – thread flank angle 60 degrees.

NPT threads are NOT INTERCHANGEABLE with BSP threads.

Common thread types include:


NPTF (Dryseal)

How it seals

Sealing is as follows:

a) When using a fixed female port with taper threads sealing is assured through deformation of the mating threads, a thread sealant is required to provide the seal on NPT threads. NPTF threads are ‘dryseal’.

When NPT threads are fully assembled without a sealant they leave a spiral leak path at the thread root/crest. NPT therefore require a suitable sealant.

NPTF threads are classed as ‘dryseal’ as when assembled they do not leave such a leak path. This is due to the fact that the thread has controlled thread truncation at the crest and root, thus ensuring full metal-to-metal contact as the male and female thread flanks make contact.

Further tightening ensures that the thread crest flatten out until the flanks also make metal-to-metal contact.

Theoretically, this means that there is no passage left for the fluid to leak through. However, for this to happen it would mean that the threads have to be free from excessive manufacturing defects and also be dimensionally exact. A thread sealant/lubrication is often used to achieve a leak-free joint when using NPTF threads.

Even after being tightened, NPT threads leave a spiral leak path as there is no thread crest and root contact.

With NPTF threads, even at hand tight there is contact at the thread crest and root.

Once NPTF threads are wrench-tightened there is thread contact at the crest and root and also at the flank.

Because of the higher surface contact and additional friction forces with the NPTF design NPT threads are often used; particularly for Stainless Steel components as the NPT thread form reduces the possibility of thread galling during installation.

More information

For more information on the‘Small Bore Tubing Integrity Course – using twin ferrule compression fittings’, please contact the Association at: For general enquiries, please contact:, or Tel: 01608 647900.