An Introduction to Fittings: Identifying Thread Size and Pitch
July 17, 2019 | Andy Hitchcock, Product Manager, Field Services
The health of your industrial fluid systems relies on every component working together to transport your process fluid to its destination. Your facility’s safety and productivity depend on leak-free connections between your components. Identifying the right fitting for your fluid system starts with understanding and identifying thread size and pitch.
Thread and End Connection Basics
Even the most experienced professionals can sometimes have difficulty identifying threads. It is important to understand general thread and end connection terminology and standards to help classify a specific thread.
- Thread Gender: Male and female threads refer to the placement of the threads on your fitting. Male threads project on the outside of the fitting, while female threads are found on the inside of the fitting. Male threads are inserted into female threads.
- Pitch: Pitch refers to the distance between the threads. Pitch identification is reliant on the specific thread standard, such as NPT, ISO, BSPT, etc. Pitch can be expressed in both threads per inch as well as in millimeters.
- Crests and Roots: Threads have peaks and valleys, referred to as crests and roots. The flat surface between the crest and the root is called a flank.
Identifying Thread Types
The first step toward identifying the thread size and pitch is to have the right tools, including a caliper, pitch gauge, and thread pitch identification guide. Use these to determine whether the thread is tapered or straight.
Straight threads, which may also be referred to as parallel threads or mechanical threads, are not designed to seal, but rather to hold a nut onto the body of a tube fitting. They must rely on other factors, such as a gasket, O-ring or metal-to-metal contact, to create a leak tight seal.
Tapered threads, which may also be referred to as dynamic threads, are designed to seal as the flanks of the male and female threads are drawn together. A thread sealant or thread tape is required to fill in the gaps between the crest and the roots to prevent system fluids from leaking at the connection.
Tapered threads are constructed at an angle in relation to the centerline while parallel threads are parallel to the centerline. Use a caliper to measure the male or female thread’s crest-to-crest diameter on the first, fourth and last full threads. If the diameter increases on a male end or decreases on a female end, the thread is tapered. If all the diameters are the same, the thread is a straight thread.
Measuring Thread Diameter
Once you identify whether you are working with a straight thread or a tapered thread, the next step is to determine the thread’s diameter. Once again, use the caliper to measure the nominal male or female thread diameter from crest-to-crest. For a straight thread, measure any full thread. For tapered threads, measure the fourth or fifth full thread.
The diameter measurement obtained may not be the same as the listed nominal size for the given thread. This variation is due to unique industry or manufacturing tolerances. Use your fitting’s manufacturer’s thread identification guide to determine the diameter is as near to the proper size as possible.
Determining Thread Pitch
The next step is to determine the thread pitch. Use a pitch gauge, also called a thread comb, and check the thread against each form until you find a perfect match. Some fractional and metric thread forms are very similar, so this may take a little time.
Establishing Thread Standard
The final step is to establish the thread standard. Once you have determined your thread’s gender, type, nominal diameter and pitch, you can identify the thread’s standard using a thread identification guide. Swagelok’s Thread and End Connection Identification Guide is an essential resource for determining your thread standard and identifying your end connection.
For more information regarding thread identification, preparation and installation, consider registering for a Swagelok Essentials training course taught by Swagelok-certified trainers at a location convenient to you. Contact your authorized sales and service center to learn more.