Nice Looking Threads, but What Size Are They?
by Katie Dennis, on Thu, Aug 06, 2015 @ 09:08 AM
Stop by Edmonton Valve's back counter so we can help you identify the thread pitch and type
Unable to identify the type or thread pitch of your component? Come visit one of our associates here at Edmonton Valve and we can assist you. And to further assist you along the way, download a copy of Swagelok's Thread and End Connection Identification Guide here.
A threaded component will fit into another threaded part only if the threads are the same size and type. If you aren't sure what size you are working with, our staff will be glad to help you get the information you need. How do they do it? Here are the basics:
Know your terms
The first thing to know about threads is that they can be straight or tapered. On straight threads, the ridges, or crests, at the front are exactly the same diameter as the ones in the back. With tapered threads, one end is narrower, like a cone. In fact, a tapered thread is essentially a wedge wrapped around a cone. If you are working with Swagelok parts, we stamp some of our products with certain thread types to help you identify them.
Some people are surprised to learn that straight threads don't provide a seal by themselves. All they do is make sure the two parts are solidly engaged. To get a seal, you need an additional component such as an O-ring, a gasket, or metal-to-metal contact with a washer.
There are several straight thread standards. The most common are SAE (Society of Automotive Engineers), ISO 228/1 (also known as British Standard Pipe Parallel), and metric.
With tapered threads, the seal is formed when the flanks of the thread engage. Even so, you also need a sealant to ensure a leak-free seal and to prevent galling of the metal.
As with straight threads, there are several standards threads conform to: NPT (National Pipe Thread), ISO 7/1 (also known as British Standard Pipe), and metric. In Canada, most threads will be NPT.
By the numbers
We have a couple of tools to help us identify a thread. First we use a set of calipers to see whether we are dealing with straight thread or tapered thread, and the diameter.
Next we use a thread comb. That's a tool that looks a bit like a pocketknife, except instead of cutting blades it has a series of saw-toothed blades. The teeth come in various pitches that correspond to the number of threads per inch on a part.
We start by simply picking a set of teeth that seem close to the threads we are trying to match. If we assess it right the first time, great. If not, it's easy to see if we need to switch to a finer or coarser pitch. For tapered threads, we take the middle point. Usually it takes only a few tries to get a perfect match, where the gauge doesn't rock back and forth.
We also have the Swagelok Thread and End Connection Identification Guide close at hand. Based on the nominal thread size, this booklet will tell us the correct designation such as NPT or SAE, the nominal thread diameter in inches and millimeters, and the pitch. The book quickly narrows down the choices by part number.
We have a cabinet with samples of adapter fittings, so we can usually make a direct comparison with a known pitch and diameter.
Knowledge is power
Naturally, we'll try to find exactly what you need in our parts warehouse. If we don’t have the exact part, we may be able to put together a component and a tube adapter that gets the job done. And even if we can't come up with the part or parts that solve your problem, we can at least send you out into the world knowing what to ask for. If you've been tearing your hair out trying to figure out what kind of component you're holding, that's a big step forward.
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