Share this
There's Nothing Like a Set of Well-Fitting Threads
by Jeff Hopkins on 3/30/16 8:00 AM
Here's how to make sure you'll always pick the thread with the right size and pitch
Swagelok fittings, valves, regulators and other components have one thing in common: threads. If the threads on two parts don't have the identical diameter and pitch, you can't connect them. So how can you tell if everything is the right size? We're glad you asked.
The basics
Threads have peaks (called crests) and valleys (called roots). The part in between the crest and the root is called the flank.
Threads also come in two basic types, straight or tapered. On straight threads, the crests are all the same diameter. With tapered threads, the crests at one end have a smaller diameter, like a wedge. In fact, a tapered thread is essentially a wedge wrapped around a cone. If you are working with Swagelok parts, it's easy to tell the difference: All fittings are stamped to tell you if they are tapered or parallel.
But you also can check it yourself. Use calipers to measure the outside diameter on the first, fourth, and last full threads. If you get three different measurements, you have tapered threads. If the measurements are the same, you have straight threads.
Here's the pitch
The space from crest to crest is the pitch, which usually is expressed as threads per inch. Straight threads come in three standard kinds of measurements. The most common are SAE (Society of Automotive Engineers), ISO 228/1 (also known as British Standard Pipe Parallel), and metric.
Tapered threads also have several standard forms of measurement: NPT (National Pipe Thread), ISO 7/1 (also known as British Standard Pipe), and metric.
The flanks can come in different angles, known as Whitworth, unified or metric.
Some people are surprised to learn that straight threads are not designed to seal on their own. They require a gasket, O-ring or some kind of metal-to-metal contact to finish the job.
Tapered threads are designed to seal as the mating threads are drawn together. In addition, some kind of sealant is necessary to prevent leaks. That's usually PTFE tape or a product such as SWAK.
Take a look
So let's put all that information together. Here's a great video to show you how it's done. You'll need a caliper, a pitch gauge, and a Swagelok Thread and End Connection Identification Guid (which you can download for free).
Use the calipers to confirm that you have straight or tapered threads. The pitch gauge has a lot of different saw-tooth blades that fit into the thread roots. Try different blades until you get the exact match. Some fractional and metric thread forms are very similar, so take the time you need to do it right. The Swagelok Thread and End Connection Identification Guide has illustrations to help you.
We're here to help
You don't have to figure it all out on your own. Swagelok Northern California has Thread Identification Centers at both of our Will Call locations. The displays have examples of threads in a wide variety of configurations, plus photos illustrating how to identify thread type and how to measure.
If you've been staring at some threads, wondering what exactly you have, we can help you get a definitive answer.
Additional Resources
Share this
- Archive (465)
- Assembly Services (207)
- About (100)
- Seal Support Systems (96)
- Best Practices (88)
- Training Services (74)
- Fittings (51)
- Semiconductor Applications (49)
- Hoses and Flexible Tubing (47)
- Regulators (44)
- Tubing (42)
- Grab Sampling Systems (32)
- Sampling Systems (32)
- Gas Systems (30)
- Services (30)
- Downloads (29)
- Valves (24)
- Application Support (18)
- Orbital Welding (17)
- Case Studies (13)
- Steam Systems (13)
- Frequently Asked Questions (12)
- Tools (12)
- Measurement Devices (7)
- Subsystems (6)
- Thermal Management (6)
- September 2023 (1)
- August 2023 (2)
- June 2023 (1)
- March 2023 (3)
- February 2023 (3)
- January 2023 (4)
- December 2022 (4)
- November 2022 (4)
- October 2022 (4)
- September 2022 (1)
- August 2022 (3)
- July 2022 (2)
- June 2022 (4)
- May 2022 (1)
- April 2022 (2)
- March 2022 (1)
- February 2022 (2)
- January 2022 (3)
- December 2021 (1)
- November 2021 (6)
- October 2021 (6)
- September 2021 (8)
- August 2021 (4)
- July 2021 (3)
- June 2021 (6)
- May 2021 (6)
- April 2021 (7)
- March 2021 (5)
- February 2021 (4)
- January 2021 (6)
- December 2020 (5)
- November 2020 (6)
- October 2020 (6)
- September 2020 (8)
- August 2020 (7)
- July 2020 (8)
- June 2020 (8)
- May 2020 (6)
- April 2020 (9)
- March 2020 (7)
- February 2020 (10)
- January 2020 (21)
- December 2019 (23)
- November 2019 (21)
- October 2019 (22)
- September 2019 (21)
- August 2019 (22)
- July 2019 (23)
- June 2019 (20)
- May 2019 (23)
- April 2019 (22)
- March 2019 (21)
- February 2019 (20)
- January 2019 (21)
- December 2018 (14)
- November 2018 (19)
- October 2018 (23)
- September 2018 (17)
- August 2018 (29)
- July 2018 (11)
- June 2018 (6)
- May 2018 (5)
- April 2018 (4)
- March 2018 (5)
- February 2018 (3)
- January 2018 (3)
- December 2017 (2)
- November 2017 (4)
- October 2017 (3)
- September 2017 (2)
- August 2017 (6)
- July 2017 (4)
- June 2017 (4)
- May 2017 (4)
- April 2017 (3)
- March 2017 (4)
- February 2017 (3)
- January 2017 (3)
- December 2016 (3)
- November 2016 (3)
- October 2016 (3)
- September 2016 (5)
- August 2016 (5)
- July 2016 (4)
- June 2016 (5)
- May 2016 (3)
- April 2016 (4)
- March 2016 (5)
- February 2016 (11)
- January 2016 (1)
- December 2015 (3)
- November 2015 (4)
- October 2015 (3)
- September 2015 (4)
- August 2015 (4)
- July 2015 (8)
- June 2015 (5)
- May 2015 (3)
- April 2015 (4)
- March 2015 (4)
- February 2015 (3)
- January 2015 (4)
- December 2014 (2)
- November 2014 (3)
- October 2014 (4)
- September 2014 (4)
- August 2014 (4)
- July 2014 (5)
- June 2014 (4)
- May 2014 (4)
- April 2014 (5)
- March 2014 (4)
- February 2014 (3)
- January 2014 (4)
- December 2013 (5)
- November 2013 (3)
- October 2013 (4)
- September 2013 (3)
- August 2013 (5)
- July 2013 (5)
- June 2013 (5)
- May 2013 (3)
- April 2013 (6)
- March 2013 (4)
- February 2013 (4)
- January 2013 (8)
- December 2012 (4)
- November 2012 (6)
- October 2012 (6)
- September 2012 (4)
- August 2012 (4)
- July 2012 (4)
- June 2012 (4)