Need Help With Tubing Thread Size

[scoostraw]

Forgive me if I am asking this in the wrong place.

I have a project I am working on where I am using 1/4" OD steel tubing. The wall thickness of the tubing is enough that it can be threaded on the outside.

I want to go from this tubing to 1/4" NPT. How would you guys go about this? I cannot seem to find anything in the way of pipe thread reducers anything close to this size.

Thank you!

 

[JimDawson]

Normally for 1/4'' tubing you would use a flare or compression fitting. There is no pipe thread that small. From the flare, fittings are available to get to any standard pipe size.

https://www.mcmaster.com/flares

 

[scoostraw]

Thank you Jim. This explains why I cannot find a pipe thread that size.

Would this be an inverted flare as is used on vehicle brake lines?

 

[hman]

¼" tubing, even heavy wall, is pretty marginal as far as ¼" threads are concerned. Even if you can cut threads, the remaining wall will be pretty thin, and likely to fail if any side force is applied. In any case, I'd suggest using ¼-28, rather than ¼-20. Then get a brass ¼" NPT plug from Home Depot, Lowes, or a hardware store. Drill and tap ¼-28 for the tubing.

That said, ¼" screw threads, being level, will NOT seal against pressure! Pipe threads are tapered, so that they can be tightened to form a seal.

What's your application?

 

[JimDawson]

Thank you Jim. This explains why I cannot find a pipe thread that size.

Would this be an inverted flare as is used on vehicle brake lines?

The inverted flare would be my first choice because I have the double flare tool. It really depends on the application. For hydraulic systems for instance, there are compression fittings that work very well.

 

[RJSakowski]

There is a 1/16-27 NPT thread. As I recall, this size was used for some old automotive fittings. Both MSC and McMaster Carr sell taps and dies for same.
Edit: I looked up the dimensions of the 1/16 NPT thread and the major diameter is .313" so it won't work in your application.

 

[scoostraw]

¼" tubing, even heavy wall, is pretty marginal as far as ¼" threads are concerned. Even if you can cut threads, the remaining wall will be pretty thin, and likely to fail if any side force is applied. In any case, I'd suggest using ¼-28, rather than ¼-20. Then get a brass ¼" NPT plug from Home Depot, Lowes, or a hardware store. Drill and tap ¼-28 for the tubing.

That said, ¼" screw threads, being level, will NOT seal against pressure! Pipe threads are tapered, so that they can be tightened to form a seal.

What's your application?

Yes that is my problem - it must seal up to about 25psi or so.

I am making a power bleeder adapter for a vehicle master cylinder. One commercial version I have is in the atached photo. The tubing is 1/4" OD - the fitting they used has a .307-32" thread. It is designed to fit a Schraeder valve, which I believe has a .305-32" thread. The difference I'm guessing is what gives it a tight seal. Unfortunately the only thing I can find with that .307-32 thread seems to be that one fitting.

 

[scoostraw]

There is a 1/16-27 NPT thread. As I recall, this size was used for some old automotive fittings. Both MSC and McMaster Carr sell taps and dies for same.

Oh... this is interesting... thank you.

 

[RJSakowski]

Swage Loc makes a very secure compression fitting for 1/4" tubing. I have used them on pressures to several thousand psi and for high vacuum work. There are other manufacturers using a similar design.

Silver soldering or brazing an adapter on would be another option.

 

[hman]

Silver soldering or brazing an adapter on would be another option.

Very good point. And for scootstraw's application (brake bleeder), even soft solder will hold the pressure he'll be using.

Back in the early '70s I worked as an instrument fitter at oil refineries, etc. in west Texas. Installed a TON of Swagelok fittings. Don't recall ever using any other brand. Loved 'em then, and still do. Only trouble is the price. But you get what you pay for!