Marking of bolts and nuts: what do the numbers, letters, serifs on the caps mean - an error in decoding is expensive

Our man first messes up, and then begins to figure out what exactly he did wrong. This is exactly what happened to me - I got stupidly for 200 bucks. But now I understand the marking of inch and metric bolts and nuts, decoding strength classes and more. I am sure that after spending a couple of minutes reading, you can avoid mistakes in the future.

Inch bolt marking

Inch thread differs from metric thread in profile, angle and pitch of turns. Do not screw a metric bolt into an inch hole and vice versa. You can, of course, twist with a strong desire, but only once.

Sometimes they think that inch is pipe threads, and metric is "car". But this is not the case.

How to distinguish inch bolts

Strength class of bolts with inch thread is marked on the head with risks. I will give an example of the marking of American SAE threads.

How to distinguish inch nuts

American inch fasteners are marked with risks, and metric numbers.

Metric bolt markings

When choosing fasteners, not only the type of thread profile is important, but also the strength of the metal. This parameter should not be neglected in mechanisms that operate under load. We are used to the fact that the lone number on the head of a carbon steel bolt is the strength class. But different standards have different labeling requirements.

High-strength bolts according to GOST 22353-77

Many are accustomed to the fact that a two-digit or three-digit number separated by a dot is a strength class. But not in hardware according to this GOST.

1. Manufacturer's stamp.
2. Metal resistance. It is this indicator that characterizes the strength class. The figure is the value in MPa divided by 10.
3. Climatic performance. The ХЛ brand is designed for cold climates.
4. Heat number (NOT STRENGTH CLASS!).

High-strength bolts according to GOST R52644-2006

In the marking of bolts according to this GOST, the strength class looks more familiar.

1. Manufacturer's stamp.
2. Swim number.
3. Strength class.
4. S - high strength hex bolt.
5. CL - designed for cold climates.

What else you need to know about bolt and nut labeling

If you have to fix a shelf in the garage or a crossbar to a fence, it makes no sense to understand all these figures with numbers. It is enough to go to the nearest hardware store and buy fasteners suitable for the size.

But if you change the bolts in some kind of equipment or mechanism operating under loads, the marking matters. Better to find fasteners with exactly the same markings. If the same is not there, then understand the standards, decoding letters, numbers, etc. Intuitively correlating brands of different standards is a bad job.

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