BSPT threads interchangeable with NPTF threads 1

[fireguy]

I am looking for technical information on why BSPT threads in 3/4" and NPTF threads cannot be used together. I have been told and read a lot of info that says we cannot use them, but nothing that tells me why. Certain people in my company says it can be done and is no big deal. We mass produce parts going to Europe and they want to do it on a regular basis.

 

[alliedcp]

If I am not mistaken i believe the difference is in the number of threads per inch. For instance 1/4 bspt has 19 threads per inch where 1/4 nptf has only 18. These threads cannot be connected due to this factor.

Ken

 

[EnglishMuffin]

Well, they both have 14 threads per inch, but BSP and BSPT threads have a 55 degree thread form, versus the 60 degree thread form of US threads, and I think there are slight differences in the major diameters. Doesn't sound like a good idea to me for official business, but its the sort of thing that maintainance guys can get away with sometimes.

 

[phfau]

Only 1/2'' and 3/4'' screw into each other, the difference is the 55` compared to 60` angle of the thread.These two sizes are the only ones with the same TPI.We do interchange these fitting but if you are a manufacture i would recommend making them to spec because you can get caught out by a good QA system.

 

[dragonsaver]

If the parts are plastic, PVC, CPVC etc then the material is soft enough that you do not need to retap the threads. This is only good for up to 1/2", It may work for 3/4" but definitely not larger sizes. If Part is metal you will need to retap the threads.

 

[M01]

Do not try to couple NPT fittings with BSPT (British Standard Pipe Tapered) fittings. Can we spell liability? NPT thread forms are 60 degrees While BSPT's (also called ISO/R7) are 55 degrees. These will not seal properly, the threads will be deformed if you try to seal them. In addition the only pitch that coincides is 14 T.P.I., some of the others are very close (ie. 1/8-27 NPT vs. 1/8-28 ISO/R7) and will cross-thread easily with the persuasion of a little elbow grease, especially in softer materials such as brass and plastics. Be aware also that ther are non-pressure pipe fittings with a 55 degree thread form with the same pitches as BSPT threads carrying the nomenclature BSPP (British Standard Parallel Pipe) or ISO/228, which are not tapered threads and not designed for pressure sealing.

 

[RobWard]

We had a bulk silicone oil spray system installed in our factory. They (the manufacturers of the system) mixed BSP and NPT threadforms. Result: lots of puddles of silicone oil throughout the facility. They argued that the threads were close enough and some PTFE tape would seal it just fine.

We ripped the system out.

From my experience, never mix different threadforms: each has been carefully designed to fulfil a specific requirement and mixing them just leads to trouble.

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams