FTM 155: ASME B1.3 System 22 Thread Gauging

Fastener Training Minute episode 155, Tri-Roll measurement gauge

This edition of the Fastener Training Minute with Carmen Vertullo was originally published August 15, 2020 as “System 22 thread gauging or thread inspection” during episode 155 of Fully Threaded Radio.

I got a question or a suggestion recently from one of our good friends, Baron Yarbrough from Spring Bolt & Nut. Baron asked if I had ever done a training minute on System 22 thread gauging or thread inspection. It seems like we might have but if we did it’s been a long time and the question comes up pretty often. So even though there is a lot more to know about the various systems of thread gauging than we can do in the Fastener Training Minute, we can get you well informed in a few minutes regarding what to do when you see System 22 gauging on a drawing or a standard.

Well, sometimes we see a notation that says thread gauging requirement System 22, and we don’t know what to do with that or how to quote that or where it comes from and or things like that. So we really it’s just like any other thing we would quote. We need to know what it means before we start moving forward with it. So a good place to start is our IFI Inch Fastener Standards 2018 Edition book or the online IFI Standards, you’ll notice there are actually four thread standards in there.


There’s ASME B1.1 which basically is the dimensional requirement for inch screw threads and there is also a B1.1M which deals with metric threads. But in this book, it’s only going to be inch threads and it essentially says here’s the design of the thread and here’s what all the dimensions for the various features of the thread would be. The major diameter, the pitch diameter the minor diameter, the thread angle and so on. Then right after that comes ASME B1.2 which tells us what the requirements are for gauges.


The title of ASME B1.2 is Gauges and Gauging for Unified Inch Screw Threads, and it essentially describes all the various types of gauges that might be available to measure various thread features. If we go one step further, we get to ASME B1.3.


ASME B1.3 is where the actual standards for gauging come from. Thats where System 21, System 22 and System 23 come from.
System 21 is the simplest, System 22 is more complicated, and System 23 is so complicated it’s the way off the charts. The book doesn’t even include system 23 because we virtually never see that requirement in any kind of commercial fastener. In fact we rarely even see it in aerospace fasteners. System 23 is for super heavy critical requirements, maybe certain types of gauges and certain types of research and development projects. We don’t see System 23 very much in production fastening.

So let’s take a look at System 21 and System 22. Incidentally, there’s a fourth standard in the IFI book called IFI 301 and it talks about gauge calibration procedures for thread gages. It hasn’t always been in the book. I recommend you read it. If you are dealing with thread gauges at all. It’s informative, but it’s not what we’re talking about here.

System 21

So let’s start with System 21 from ASME B1.3; it’s the simplest system and that’s what we’re kind of used to measuring our threads with. It involves using “go” and “not go” gauges such as ring gauges for externally threaded fasteners and plug gauges for internally threaded fasteners.

We use plain plug gauges to measure the minor diameter of the internal threads on nuts, and we use calipers or micrometers to measure the major diameter of the threads on externally threaded fasteners. That’s pretty much about all that’s required to be measured under system 21 and if we look in system 21 will see there’s a list in a table that tells us which things we need to measured and also which gauges are allowed to be used to measure them. We have some other things that we have to measure besides major diameter and functional size with the ring gauge. When we are using a ring gauge we’re really not measuring any particular thread, we’re measuring a bunch of threads all put together. We don’t really know what size the thread is; all we know is that it’s between its minimum and its maximum in terms of what the tolerance is. That’s what a ring gauge or plug gauge will give us.

System 22

When we step up the System 22, we still want to know what the maximum material condition of the screw is, and we can still measure that with a ring gauge or a plug gauge. But we also have a requirement to measure something called pitch diameter. Pitch diameter deals with one thread only; a single thread. We’re going to measure one thread and we need a special tool to measure just one single thread. Typically that tool is going to be what we generically refer to as a Tri-Roll Gauge and there other types of those called Segment Gauges. If we are talking about a nut and we are measuring an internal thread, the measuring tool is called to Bi-Point gauge. Bi-Point Gauges have got these little tiny legs with little teeth or feet on the end of that go in and contact the internal thread gauge or the internal thread. And that gauge is connected to a gauge that gives us a readout, and when we have a gauge that has a readout we call that a Variables or Indicating gauge where we actually get a number.

Those other gauges, ring gauges and plugs gauges are called Fixed Limit Gauges, and those only measure what are called attributes or if the measurement is either “in spec” or “out of spec”. So in System 22, we have the more complex requirement where we have to measure the thread pitch diameter one thread at a time using a Variables Gauge. Now we can also measure the maximum material condition with variables gauging. The difference is that when we look at our maximum material condition, our thread rolls on the Tri-Roll Gauges have many grooves in them. So these gauges see the same thing that the ring gauge would see, except we get a number.
Even if we are not required to get a number, we’re okay using a ring gauge on system 22, but we have to use the Variables Gauge to get the pitch diameter; there’s no other reasonable way to get it. But there is still a good reason to use Variables Gauging even for system 21 if we are using Tri-Roll gauges, multi Rib-Roll Gauges or single Rib-Roll Gauges to measure pitch diameter. And that good reason is it’s fast. It’s much faster to measure threads using Variables Gauges than it is to use Fixed Limit Gauges because you don’t have to spend that thing up and down. Also you’re getting your “go” and “not go” with one measurement because you’re getting a number instead of having to use a “go” gauge and and “not go” gauge. You just use the variables measurement like you were doing system 22, even though the requirement is for System 21. Now that all sounded a little bit complicated and if you are confused, I have some very good documentation on this from Larry Borowsky of Greenslade & Company that I’d love to share with you if you need it.

The last thing we need to know is when do we have to use System 22 VS System 21. Well, first of all, all the ASME product standards that have screw threads will indicate which system is required to be used. Those are dimensional standards are going to be somewhere in a table or a footnote or notes to a table in an ASME standard. The note will say “gauging shall be ASME B1.3 System 21”. Almost every single standard that we have specifies System 21; the only exception that I’m aware of is a ASME B18.3 for socket head cap screw. That specifies System 22 gauge and if you’re in the socket screw business, making socket set screws or making socket cap screws you’re required to do that. Another thing is if you’re a manufacturer you want to be using variables gauging anyway, because it helps you keep your process under control, and if you’re using any kind of statistical process control, you got to have variables gauging on your shop floor. The other area where we commonly see system 22 gauging is in any kind of Aerospace. If our world is inch socket head cap screws and Aerospace and some military standards then that’s where System 22 gauging and comes in.

There are a few other details that go along with System 22, things like measuring the the thread root profile, and we have to do that with something like an optical comparator. Actually even System 21, we sometimes have to measure the minor diameter of a rounded root profile, but we only have to measure the diameter not the actual radius.

So those are things that are different between System 21 and System 22. It was a little bit more complicated to explain than even I thought going into it, but I want to thank Baron Yarborough for the question. As a matter of fact in our AIM Fastener Standards Review Channel, we’re almost finished with quality standards, and I think the next thing we might go into might be thread standards.

So we’ll just see what the audience thinks about that and it will be quite a number of episodes because it’ll take some time to get through those three standards.

Well, that’s all I’ve got on System 22 thread gauging on theFastener Training Minute, actually a little bit more than one minute. I hope you enjoyed it.

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