In Part 1 of our Threads Series, we provided some terminology and explained some of the nomenclature of Unified Inch Series threads. In Part 2, we talked about the difference between 2A and 3A threads, again, in Unified Inch Series threads. Now, here in Part 3, we’ll be covering the basics of metric threads. As usual, we here at Wilson-Garner manufacture externally threaded parts, so we’re going to primarily examine external threads.

How to Speak Thread – Metric Version

As we did with inch threads, let’s start with an example that will provide some insight into how we express metric threads:

Close up of metric threads

This photo is of a metric bolt with M14 x 2.00 6g threads. Let’s talk about what each part of that nomenclature means.

“M14” tells you that the nominal major diameter of the threads is 14 millimeters. Major diameter is the diameter as measured at the widest part of the threads. “Nominal” means “basically”. The specs on this part actually call out a major diameter of between 13.682 and 13.962 millimeters, but nobody wants to say “M13.962”, so we just say “M14” and move on with our lives.

The “2.00” is in reference to thread pitch. You’ll recall from our first article that thread pitch is the distance from one point on a thread to the same point on the next thread. Here, the “2.00” tells you that the thread pitch is 2.0 millimeters. If you look at the photo above, you can see this to be true, as there’s 2 millimeters between the crest of one thread and the next. This is a major difference from Unified Inch Series nomenclature, where the second number calls out threads per inch and not thread pitch.

Similarly to Unified Inch threads, metric 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. 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 fact, in some specifications, the pitch is not even called out in the product description.

Metric Thread Fit

The final piece of nomenclature above, “6g”, refers to the metric version of thread fit. In Part 2 of our Thread Series, we talked about thread fit in the Unified Inch system, specifically, 2A and 3A thread fit. In metric, 6g is roughly comparable to 2A in the inch system. 6g is a short form expression for the external thread fit tolerance class 6g6g. 6g6g thread fit is quite common. Let’s examine the terminology. The left number/letter combination specifies the pitch diameter tolerance and allowance, while the right number/letter lays out the tolerance and allowance for the major diameter. The numbers specify tolerance, while the letters specify allowance.

What are tolerance and allowance? In case you didn’t read Part 2, I’ll restate it here: Thread fits are developed using allowances and tolerances. An allowance is an intentional clearance between mating threads. Allowances are applied to external threads. The major, pitch, and minor diameter maximums are less than the basic size by the amount of the allowance. Tolerances are specified amounts by which dimensions are permitted to vary for manufacturing convenience. Tolerance is the difference between the maximum and minimum permitted limits for a given dimension.

Some Examples

Before we get too far into the weeds here, we should get some visual aids going. Let’s look at an example of a very popular size: M10 x 1.50 6g6g:

Diagram of an example M10 x 1.50 6g6g Thread showing allowance and tolerance

Remember, the leftmost number/letter specifies the pitch diameter requirements, while the right number/letter spells out major diameter requirements. So in this case, both the pitch diameter and major diameter have a tolerance grade of 6 and an allowance of g. Here’s what this translates to in actual numbers:

Pitch Diameter – For M10 x 1.50 6g, 6 is 0.132mm tolerance, and g is 0.032mm allowance.

Major Diameter – For M10 x 1.50 6g, 6 is 0.236mm tolerance, and g is 0.032mm allowance.

Notice that the major diameter has more tolerance given than the pitch diameter, while the allowance is the same for both.

Changing the Tolerance

Now, a second example, where we have a thread of M10 x 1.50 4g6g:

Diagram of an example M10 x 1.50 4g6g Thread showing allowance and tolerance

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.

Pitch Diameter – For M10 x 1.50 4g, 4 is 0.085mm tolerance, and g is 0.032mm allowance.

Major Diameter – For M10 x 1.50 6g, 6 is 0.236mm tolerance, and g is 0.32mm allowance.

By changing only the first number, we have tightened the tolerance on the pitch diameter and left everything else the same.

Changing the Allowance

Moving on, we’ve got another example. This time, we’re changing the letters – M10 x 1.50 6e6e.

Diagram of an example M10 x 1.50 6e6e Thread showing allowance and tolerance

As letters decrease, 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. Here’s the numbers:

Pitch Diameter – For M10 x 1.50 6e, 6 is 0.132mm tolerance, and e is 0.067mm allowance.

Major Diameter – For M10 x 1.50 6e, 6 is 0.236mm tolerance, and e is 0.067mm allowance.

A note on allowances: as letters increase, the amount of allowance decreases, with the “h” designation having no allowance.

A Side-by-Side Comparison

For our final example, we will look at two thread fit classes side by side. Figure 4 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). Because of the tighter tolerance, 6H/4g6g is a tighter fit than 6H/6g6g.

Side by Side comparison of 6H/6g6g and 6H/4g6g

And that concludes our talk about metric threads. We hope this has been a useful primer for you. If you’d like more information, we recommend you take a look at the following standards:
ANSI B18.2.3.1M
ISO 4014 and ISO 4017
GMW 16551
DIN 931 and DIN 933

Thanks as always for reading.

Further Reading

In addition to our Threads Series, we’ve got plenty of other articles covering fastener basics. Here’s a sampling.
What is Torque-Tension Testing?
Bolt & Screw Head Markings and What They Mean
Different Steel Grades and Hardenability
Cold Heading Vs Hot Forging Vs Machining
The Difference Between a Bolt and a Screw

Or, check out our full list of articles.