4/20/23, 12:28 PM
Thread Inspection 101 Part IV - Thread Designations (Metric) | Fox Valley Metrology
Thread Series
This is the designator specifying that it is a part of the Metric series, much like the "UN" in the inch
system. This will always be start with an "M".
As with the Unified Thread System, additional modifiers can be added here to tweak how the thread is
machined. Some common examples are:
M - Basic thread profile as specified in ISO 68 (https://www.iso.org/obp/ui/#iso:std:iso:68:-1:en)
MJ - MJ profile with round root on external threads and thread ring gage
MJS - MJ profile special series on external threads and thread ring gage
Nominal Diameter
Much like in the inch system, the nominal diameter is the theoretical diameter from which the design size
limits are derived by the application of tolerances and allowances. The primary difference here is this
diameter is given in mm, not inches.
Thread Pitch
The first major variance in the designation system comes in the form of the thread pitch designation.
While this still calls out the thread spacing, it does so in the opposite fashion of the Imperial system.
In the Metric designation system we define this by pitch, the inverse of threads per inch, as is given in the
Imperial designation system. This pitch is also given in mm.
Class of Fit
Much like the numeric classes of fit used in the inch system, this number denotes the acceptable
tolerance class of a metric thread specification. There are 5 common classes, in order for most precise to
most loose, with class 6 being the general use designation:
4 - Very accurate applications. Available for both internal and external threads.
5 - Mostly accurate applications. Available for external threads only.
6 - Most common class of fit. Used for general use. Available for both internal and external
threads.
7 - Mostly loose applications. Available for external threads only.
We're Online!
8 - Very loose applications. Available for both internal and external
threads.
How may I help you today?
https://www.foxvalleymetrology.com/blog/posts/2019/july/thread-inspection-101-part-iv-thread-designations-metric/

2/7
4/20/23, 12:28 PM
Thread Inspection 101 Part IV - Thread Designations (Metric) | Fox Valley Metrology
A visual representation of this concept is shown here:
Allowance Adjustment
Perhaps the biggest deviation from the Imperial system designation is with the additional identifier for
the allowance adjustment in a metric thread specification. These designations further tweak how the
"fit" of a thread system is supposed to be.
There are four primary allowances:
E - Very loose applications for external threads.
F - Loose applications for external threads.
G - For common use internal threads and loose application external threads.
H - For common use external threads and accurate application internal threads.
Think of each of these allowance categories as "buckets". Within each of these buckets there are several
allowable classes, identified above (4-8). Therefore, when combined, these two designations allow for
very granular specification.
Internal vs. External
This designation is not done defining the thread, though. The final point it makes is whether the thread is
an internal or external thread. This is simply done through capitalization.
Uppercase - Internal Thread
Lowercase - External Thread
When you put this all together, the tolerance chart for metric gages looks something like this:
We're Online!
How may I help you today?
https://www.foxvalleymetrology.com/blog/posts/2019/july/thread-inspection-101-part-iv-thread-designations-metric/

3/7
4/20/23, 12:28 PM
Thread Inspection 101 Part IV - Thread Designations (Metric) | Fox Valley Metrology
Modification
Similar to the UTS designation, the metric thread designation system allows for additional modification of
the standard specification.
The most common way this occurs isby adding a second class to the class of fit designation. When this
is done, the first class of fit is being used to describe the tolerance applied to the pitch diameter of the
thread and the second is being used to describe the tolerance for either the major (external threads) or
minor (internal threads) diameter of the thread.
Using our example from above, another possible example would look something like this:
M8 X 1.25 - 6H4H
In this instance, the designation is being made of the following:
6H - Tolerance of 6H for the Pitch Diameter
4H - Tolerance of 4H for the Minor Diameter
As you can see, this adds an immense level of customization to any thread designation.
In Part V of our Thread Inspection 101 series (/blog/categories/thread-gages/), we will explore the
complicated world of the theory behind thread inspection.
Interested in having your thread gages calibrated? Check out our Thread Ring Calibration Services
(/services/calibration/calibration-service-offerings/hard-gage/thread-ring-gages/) or our Thread Plug
Calibration Services (/services/calibration/calibration-service-offerings/hard-gage/thread-plug-gages/)
today!
Interested in purchasing a thread gage? Please submit a request (/products/hard-gaging/) today.
We're Online!
Posted by Steve Toll on 16 Jul 2019
How may I help you today?
https://www.foxvalleymetrology.com/blog/posts/2019/july/thread-inspection-101-part-iv-thread-designations-metric/

4/7