May 2013 Newsletter: Back to the Basics: Thread Addendum

Gagemaker Gage Report

Back to the Basics: Thread Addendum


What is Thread Addendum?

We’ve had a few questions on the subject of thread addendum, so we’d like to take a moment for clarification.

Thread Addendum : Distance from the crest to the pitch line

Thread Addendum is simply the distance from the thread crest to the pitch line. Addendum is a shorter distance than the Thread Height.

Further, Addendum is now API mandated to be measured and controlled on tubing and casing 60° V threads (8 Round,10 Round, and Line Pipe). (updated for AP1 5B 16th Ed.)

Because pitch diameter is monitored and controlled through the physical inspection of crest diameter, measuring and accounting for addendum is extremely important.

Lastly, while the thread height and thread addendum gages look similar and sometimes can be interchanged, they have different indicators, standards, and contact points. Understanding the gaging differences between thread height and addendum improves your inspection capabilities.

Notice some of the differences (contact points, indicators, basic shape, etc.) below:

How do you measure it?
Take a look at this quick video clip of a Thread Addendum Gage inspecting a part:


Gagemaker, LP develops and manufactures innovative precision mechanical inspection equipment for the Oilfield, Automotive and General Machining industries. Recognized as the leading provider of Tapered & Straight Thread Measurement Gages; API Products, Tubing and Casing Gages, Engineering Software and Calibration Software & Equipment, Gagemaker products are used in countless facilities around the world.