Can you mix thread types?

[buttert]

I was recently asked to research the possibility of mixing threads at my job. From my own experience, it's never worked, but I'm not an expert on this.

My boss wants to know if there are any circumstances where paralel threads and tapered threads can be put together, or ever BSP and regular paralel, or any crossing of threads at all...

 

[KENAT]

Not sure if it's what you're asking but some metric sizes fit in similar sized inch, or maybe the other way round. I think it's M5 and 10-24 but I can't remember for sure. It only works one way from memory, the male of one goes in the female of the other.

I wouldn't say it's good practice though.

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[MikeHalloran]

I found a BSPT gage screwed into a BSPP fitting in a boat that probably cost upward of $75m. I notified the boat's engineer, slobbered on a bunch of Loctite PST, and put it back together that way. I'd bet it still hasn't been fixed.



Mike Halloran
Pembroke Pines, FL, USA

 

[brengine]

For low-pressure applications (~100psi or less), I've been told it's acceptable to screw a Male BSPT Fitting into a female BSPP Port. The limiting factor being that there are really only a couple threads engaged (but at least they have the same pitch). Screwing a BSPP into a BSPT is not recommended.

All other mixing and matching of threads is technically cross-threading (although it may "appear" to fasten correctly)...again you're only going to get a couple threads engaging.

As for plastic tubing:
8mm = 5/16"
and
4mm = 5/32"

 

[MikeHalloran]

If it had been _my_ boat, I'd have fired someone, or tossed them overboard. The gage measured cooling water pressure on a generator engine. If it had managed to unscrew itself, a real possibility, it might have flooded _all_ the generators with seawater before anyone noticed.


Mike Halloran
Pembroke Pines, FL, USA

 

[tygerdawg]

Like KENAT said, "not good practice." My experience is sure, it can work....ONCE. But I'm conservative about these things.

However, I was recently shocked to see that NUMATICS brand is selling fittings called "WorldThread" or something like that. They are supposed to be designed to fit a wide variety of straight/tapered threads for pneumatic equipment. Reading the catalog was complicated and made my head hurt so I didn't quite get it.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[buttert]

Thanks, that confirms how I feel, that it is not ever a good practice.

 

[Compositepro]

According to the McMaster-Carr catalog male BSPT can be used with female BSPT (taper) or BSPP (parallel).

 

[juergenwt]

My advise - stay away. If and when it fails, and it will fail, you will be blamed. There is no advantage to it.

 

[mfgenggear]

buy the right threaded fasteners
or machine the correct threads.

 

[FeX32]

3 words.... don't do it!!!


Fe

 

[FeX32]

you're a jabberwock if you do...


Fe

 

[plasgears]

In the USAF they called it 'Kluge' expedient fixes. Translated, it means an expedient fix that appears sound at cursury inspection. Most USAF shops post a sign: NO KLUGES!

 

[patprimmer]

When you are 5 days walk from the nearest supply and your car is broken, lots of things will work and BSPP with BSPT either way will get you home with plenty of good thread seal tape or some Loctite 515.

I have even jury rigged with NTP into BSP. Some sizes have about the same pitch but different thread forms.

I repeat these are not recommended, but sometimes you have to do whatever it takes.

Regards
Pat
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