Tapered vs straight threads

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Old 05-30-19, 09:04 PM
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Tapered vs straight threads

Would a female npt tapered fitting make a water/gas tight seal with straight male threads if used with pipe joint compound?

or would the two not even engage with each other?

 
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Old 05-31-19, 05:15 AM
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No. There are reasons for different threads on pipe. Sealing against pressurized content is one of them. Relying on pipe joint compound as a seal for gas using pipe with two different types of thread is asking for trouble.
 
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Old 05-31-19, 05:41 AM
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I should clarify that this is a hypothetical...
 
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Old 05-31-19, 06:05 AM
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The answer is still a big fat NO. For example 1/4" straight threads are either 20 or 28 threads per inch. 1/4" NPT is 18 threads per inch. It won't even come close to sealing.
 
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Old 05-31-19, 07:15 AM
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So a leak would be immediate.

I wasnt sure if it was even possible to put them together.

Would they otherwise join together securely or would it be obvious they werent made up properly?
 
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Old 05-31-19, 09:14 AM
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You might get one started in the other but you would be cross threading. Whether or not it would be immediately noticeable depends on the person. Someone used to working with nuts & bolts would know right away that they are cross threading and would stop before damage is done. Someone less knowledgeable may just keep tightening down and wonder why it's leaking and by that point one or both pieces may be damaged.
 
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Old 05-31-19, 12:16 PM
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Rymik, instead of asking others to answer your hypothetical questions, go on the internet and do some searching about threads, Then get some piping material and set up a lab, do some experimenting, and share your results with us.
 
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Old 05-31-19, 05:57 PM
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I recently had a customer with a 1/2" NPS fully threaded (no taper) hose fitting (not a hose thread, this was full depth 1/2 pipe thread). She wanted to attach this to a pump with standard 1/2 FNPT connection. I told her it was not a true pipe connection and leakage will occur.
Since the pump and the connection was to be submerged anyways I told her would do the job she wanted. This was for an outdoor fountain.
 
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Old 05-31-19, 06:18 PM
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For those interested...

NPT is an interference fit between pipe fittings requiring thread tape or compound to complete the seal

NPTF...is for fuel purposes and does not require pipe thread compound to form a leak free seal.

NPS... is used on many electrical assemblies such as lamps that will give a full range of adjustment. Also for fluid but must have an "O" ring seal for leak free service.

All of the above share the same thread with small difference and will engage with each other seemingly to fit but are not compatible and will not be leak free..
 
 

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