Metric thread chart

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ISO Standard metric thread Chart PDF

As the numbers decrease, the amount of tolerance decreases. With a 4g6g thread fit, we have decreased the amount of tolerance for the pitch diameter and left everything else the same.

Fastener threads are essential components in precision engineering and manufacturing. Be it on a bolt, screw, stud, or nut, these threads ensure compatibility and strength in a slew of applications, from automotive to aerospace industries and more. We’ve already defined some basic fastener thread terminology and explained the nomenclatures of Unified Inch Series threads in a previous article if you’re interested in that. Otherwise, this article will solely focus on the basics of metric fastener threads.

Standard thread pitch chart

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The “6g” refers to the metric version of thread fit. 6g is the short-form expression for the external thread fit tolerance class 6g6g, which is quite common. In our article about Unified Inch threads, we talk about  the 2A and 3A thread fit. In metric, 6g is roughly comparable for 2A in the Inch system.

The diagram above shows the entire mated class of fit for 6g6g and 4g6g external threads with a class 6H internal metric thread. (Internal threads are expressed with capital letters. Just like Unified Inch threads, the internal metric threads generally have no allowance.)

Metric thread pitch chart

Standard Thread Size Chart PDF

If you’re not already familiar with some of the basic thread terms, here’s a brief overview of the most important ones you’ll need to know to understand the rest of what’s covered here.

The left number/letter combination specifies the pitch diameter tolerance and allowance, while the right number/letter combination specifies the major diameter tolerance and allowance. The numbers specify tolerance, while the letters specify allowance.

In contrast, as allowance letters increase, the amount of allowance decreases, with the “h” designation having no allowance.

Metric thread Chart PDF

Similar to Unified Inch threads, metric fastener threads can have fine and coarse pitches. In our example above, M14 x 2.00 is a coarse thread. The fine pitch would be M14 x 1.50.

Hopefully, this article has been a helpful introduction to metric fastener threads. If you’d like more information, we recommend taking a look at the following standards:

Metric fine Thread Chart

The “M14” tells us that the nominal major diameter of the threads is 14 millimeters. Remember, major diameter is the diameter as measured at the widest part of the threads, and “nominal” means “basically.” In fact, the spec on this part actually calls out a major diameter of between 13.682–13.962 millimeters but nobody wants to say “M13.962,” so we just say “M14” and move on with our lives.

Metric coarse thread chart

Disclaimer: At Wilson Garner, we mainly manufacture externally threaded fasteners like bolts, screws, and studs, so that’s where our focus is in this article. You can learn more about internal fastener threads here if you’d like.

Notice that this is a major difference from Unified Inch Series nomenclature, where the second number calls out threads per inch instead of thread pitch.

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As letters decrease (think alphabet backwards), the amount of allowance increases. So, in the above example, the tolerances for major diameter and pitch diameter are the same as 6g6g, while the amount of allowance for both pitch and major diameters has increased.

The “2.00” refers to the thread pitch, which, again, is the distance from one point on a thread to the same point on the next thread. Here, the “2.00” tells us that the thread pitch is 2.00 millimeters. If you look at the photo, you can see this to be true, as there are 2 millimeters between the crest of one thread and the next.

Remember, the leftmost number/letter specifies the pitch diameter requirements, while the rightmost number/letter specifies the major diameter requirements. So, in this case, both the pitch diameter and major diameter have a tolerance of 6 and an allowance of g.

However, in contrast to Unified Inch threads where coarse and fine pitches are fairly standard, metric fine pitches can vary depending on the specification. For the most part, coarse threads are assumed unless specified otherwise. In some specifications, the pitch isn’t even called out in the product description.

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