# Taper- and/or Straight- thread pipe and fittings



## Semper Vaporo (Jan 2, 2008)

Are you supposed to mate Male Straight threads only with Female Straight threads, and Male Taper threads only with Female Taper threads? Or, can you mix Straight and Taper threads (Male to Female and Female to Male)? 

If they can be mixed, is there a "best" way? I mean, is it best to mate Male Taper with Female Straight, or the other way around (Male Straight with Female Taper)?

I have a condition where I am going to have to make the correct length of 3/16" pipe to connect two steam ports together with two pipes and a Union.

I have to purchase the Union and it can come with either Female Straight or Taper threads on both ends (Taper is $0.25 more). I need to purchase a length of brass pipe to make the unique length of pipe to fit the space. To thread the ends of the pipe I will need to purchase a 3/16-40 Die. The Straight thread Die is $12.50, but the Taper thread Die is $30.00. (OUCH!)

I would prefer Taper threads for their superior sealing capability, but the cost of the Taper Die is a pain, considering I expect to use it only 8 times in total (granted, one never knows, but I do not foresee ever needing to put threads on this size pipe again).

So, do I buy the Taper thread Union and the Straight Die or the Straight thread Union and the Taper Die or both Straight or both Taper???? Which is "best"?

"Cost" says Straight threads all way around, or for $0.25 more I can get Taper on the Union, but I would "like" Taper on the pipe and I guess I am willing to spend the extra $14.50 to get that, but the pipe will need to go into some pre-existing Female Straight thread fittings.

Too many "buts" in my thinking... anybody got some "words 'o wisdom" about this?


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## Kovacjr (Jan 2, 2008)

The taper fittings require you to thread on until then seat where the straight you can just have a packing nut/washer to hold it in place to what you are connecting. I would make a new steam T and splitting pipes with a nut and ferrule all soldered in place so it was a perfect fit. That also keeps a good seal and simple to remove.


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## Greg Elmassian (Jan 3, 2008)

Taper threads have the advantage of being cheaper (no gaskets), but also, the alignment of parts has to have more "give" because where everything winds up is really related to how tight you make the joint. Also, over time, the taper will "wear in" with repeated disassembly and reassembly, so again alignment of what is connected will change. 

I would opt for flanges and gaskets and straight threads, but it's more work and more cost. 

I'd go taper fit if the parts do not have to be disassembled or reassembled often, the joint should be neater looking (smaller). 

Regards, Greg


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## Chris Scott (Jan 2, 2008)

Posted By Semper Vaporo on 14 Nov 2009 03:15 PM 

Are you supposed to mate Male Straight threads only with Female Straight threads, and Male Taper threads only with Female Taper threads? 


Yes

Posted By Semper Vaporo on 14 Nov 2009 03:15 PM 

Or, can you mix Straight and Taper threads (Male to Female and Female to Male)? If they can be mixed, is there a "best" way? 



The only way to mix them is to re-tap so they match. But they usually don't fit as well as T-to-T or S-to-S. I think S-to-S is better and easier. I use a small amount of thread sealant for a tight permanent seal. I leave a very thin neat bead of sealant showing, like chalking, to remind me the it's there.


Stick with one or the other. Use whatever thread type is used on the rest of the fittings on you loco, don't mix and match. If the loco now has all S-to-S then use S-to-S. Don't use one T-to-T with all the others being S-to-S. Mixing will sometime in the future conspire to drive you nuts, guaranteed!.


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## xo18thfa (Jan 2, 2008)

C.T. Our little 7.5" gauge discovered you can mix taper and straight. 3/16" and 1/4" Model Taper Pipe are 40 threads per inch. You can get 3/16" and 1/4" dies in 40 TPI straight thread. Use some Loctite #242 to seal the joints. That is a good practice all the time.


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