Metric Head Markings – Bolt & Screw Head Markings and What They Mean – Part 3

In Part 3 of our series on head markings, we’re going to talk about metric fasteners. We’ve already covered manufacturers’ head markings and fastener standards in Part 1. We followed that up with Part 2, where we covered some common standards for inch series parts. Before I get to talk about the major metric spec that we use, ISO 898-1, I need to first talk about metric fasteners in general.

It is important to be aware that metric fasteners are part of a completely different system than inch series fasteners. Metric parts have different units of measure, different threads, and different specs. You can’t “convert” parts back and forth from metric to inch series. There are rough equivalents and points of overlap – for example, an M6 and a ¼ inch bolt are roughly the same diameter and an M10X1.50 nut can be threaded onto a 3/8-16 bolt – but the systems are separate from one another. All right, let’s jump into the spec.

ISO 898-1

ISO 898-1 is the only spec that we’re going to talk about in our discussion of metric fasteners, because it is far and away the most commonly used metric spec for the type of fasteners that we’ve been talking about. This standard covers bolts, screws, and studs made from carbon steel and alloy steel, and it lays out the requirements for ten property classes. A property class (PC) is the metric equivalent of a grade in inch series terminology. Of these ten property classes, I am going to focus on three of them: PCs 8.8, 10.9, and 12.9.

One nice thing about this standard is that the property class number designations have a concrete meaning. Whereas the grades that we discussed before had somewhat arbitrary designations (an SAE J429 Grade 8 bolt is stronger than a Grade 2, but the “8” and the “2” don’t actually refer to anything, for example), the property class number designations refer to real numbers.

A property class designation consists of two numbers separated by a dot. The first number is 1/100th of the part’s nominal tensile strength in megapascals (MPa). The second number indicates one tenth of the percentage of the part’s nominal yield strength compared to the part’s nominal tensile strength. So, in English, a PC 8.8 bolt is a bolt with a nominal tensile strength of 800 MPa and a nominal yield strength that is 80% of the bolt’s nominal tensile strength, or 640 MPa. You probably noticed that I used the word “nominal” quite a bit in the last couple of sentences. That is because the strengths indicated in the numbering system are meant to be ballpark figures and not necessarily the exact required strengths.

Property Class 8.8

Let’s look at a PC 8.8 head marking:

WG1

One thing I really like about the ISO 898-1 PC designations is that the approximate tensile strength is right there on the head of the part. PC 8.8 parts have a minimum tensile strength of 800 MPa for parts 16mm in diameter and under, and 830 MPa for parts with a diameter over 16mm. To give you an idea of how this compares to the strength of inch series parts, 6.9 MPa are about equal to 1 Kpi. 800 MPa are about 155 Ksi. So a PC 8.8 bolt is a medium strength fastener that is roughly the same strength as an SAE J429 Grade 5 fastener.

Property Class 10.9

Here is the PC 10.9 head:

WG2

PC 10.9 fasteners have a minimum tensile strength of 1040 MPa for all sizes. As you can see, they are slightly stronger than the nominal value in the PC designation. PC 10.9 fasteners are considered high strength parts. They are in the neighborhood of SAE J429 Grade 8 parts.

Property Class 12.9

Finally, here’s the PC 12.9 head, and there is nothing surprising about it:

An example of ISO 898-1 Property Class 12.9 head markings on a bolt

PC 12.9 parts have a minimum tensile strength of 1220 MPa for all sizes. These parts are very high strength. In fact, they are the strongest of all the parts that we have covered in this series of blogs. 1220 MPa are roughly equivalent to 175 Ksi.

This concludes my introductory series on common head markings. I have by no means been exhaustive in my coverage, so if you happen across a head marking that you cannot identify, contact me and I’ll try and help you out. Thanks for reading.