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I'm interested in bringing in a GHT thread compatible product, but my sample's diameter was slightly smaller. GHT has a large (pitch to pitch) diameter of 1.0625 inch. The sample I received has a diameter of 1.0415 inch on the male side, and it doesn't leak and screws in perfectly.

Is this diameter difference meaningful for the long term reliability of my product?

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    $\begingroup$ Do you know what the typical allowable tolerance or design dimensions are? You are talking about small differences which could quite easily be typical variance based on the specification. $\endgroup$ – hazzey Sep 22 '16 at 18:50
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GHT relies on a gasket to seal, not a tight fit between the threads. This makes it more resistant to dirt and also cheaper to manufacture. I would guess that your sizes will work fine and you shouldn't be concerned.

From the sizes you measured, the radial clearance is only .0105 (ten thou.) That might be a little loose for precision threads, but anything smaller than about 2 thou clearance becomes very difficult to assemble.

All that said, the Machinery's Handbook has information on hose threads and gives some tolerances. It's on page 1873 (at least in the 28th edition.) Depending on the material and processes, they list a minimum major diameter for the male thread as low as 1.035. This website also lists the appropriate dimensional information in case you don't have the handbook. The official standard is ANSI/ASME B1.20.7. If you are specifying compatible threads for a production run, you should probably consider buying it or at least referencing it on your drawing.

On another type of thread, like NPTF where the threads are the sealing component, tolerances will be tighter. Fittings that have a tapered or flared metal-to-metal surface to seal usually have thread tolerances somewhere in the middle of these two.

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  • $\begingroup$ Thanks, I really appreciate your response. One followup - In the table you've linked, the first row (NH - which my factory used to refer to my product) shows the minimum major diameter at 1.0455 which is larger than my sample of 1.0415. Given the type of threading, you're still certain that it won't result in a less reliable product? $\endgroup$ – David Sep 22 '16 at 19:49
  • $\begingroup$ According to the handbook, NH would apply if it is a cut or rolled thread and NHR would apply if it is a stamped or formed thin wall material. If yours is cut or rolled (NH,) it is technically out of spec, but that doesn't necessarily mean it won't work just fine - it would depend on the details of your application. $\endgroup$ – Ethan48 Sep 22 '16 at 19:53
  • $\begingroup$ Assuming its a NH thread, and that the product is a pressure regulator that adjusts water pressure for rv's- would you feel comfortable with the aforementioned thread size? $\endgroup$ – David Sep 22 '16 at 20:25
  • $\begingroup$ That's a risk management call that only the person taking on the liability can make. I will say that if I was preparing to sell a part to the general public, I would define what the tolerance was on the drawing and I would make sure there were process controls in place to make me reasonably certain that every part with my name on it met those tolerances. But maybe that risk-aversion is why I'm not an entrepreneur. There are certainly a lot of widgets that you can't sell competitively if you hold them to strict quality standards. $\endgroup$ – Ethan48 Sep 22 '16 at 20:29
  • $\begingroup$ You're making an extremely reasonable point. Thank you very much for your assistance. $\endgroup$ – David Sep 22 '16 at 20:38

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