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by Alain Hoffmann

Nuts and Bolts

Nuts an Bolts are the most common method of holding single parts together. They are also the cause of many, many curses and swears.

Let's start with the definitions.Bolts are separated by the following physical differences:
  • Thread
  • Thickness
  • Lenght
  • Different materials used
  • Form of the head
  • Coatings
  • Securing devices
First see if it's a metric bolt or something else. For this look at the head:
right: The one on the left indicates a standard metric bolt. The right one is (in this case) an UNC-bolt
The most important is obviously the size. The overall diameter is d. The lenght is l, indicated without the head. Those 2 measurements are always indicated on the boxes.

Now you have the possibility of different tread steepnesses. On metrics you normally find only standard (coarse) treads- DIN 13T1- , fine treads - DIN13T2 to10 - are seldom found, normally in high vibration areas like cylinder heads. On non-metrics you must define which tread you have as there's a possibility of half a dozen different ones- a real can of worms. You may have Withworth with an teeth angle of 55° or UNC with an angle of 60° or UNF or even BMC...

So let's limit to metrics.


A box like this on the left means:

SKF is the manufacturer

DIN 933 means it's a standardized bolt and head. DIN 13T1 means standard coarse tread.

M10x20 means a bolt of 10mm thickness and a lenght of 20 mm.

A2-70 means it's a stainless steel bolt, class A2 and has a strenght 70 (stainless is also available as A1, A4 and A5)


How much load a bolt can take depends of the material used. It's measured at the moment the bolt starts to deform for good.(Usually that means it breaks) The force measured is compiled in Newton per square Milimeter.The second criteria is the moment at which the bolt starts to deform (but can return to it's original size). High tensile bolts start to deform at 90% or more of the maximal load. Standard bolts may already start at 60%.

The first number means a tensile strenght of 800 Newton per sqr mm for the left and 600 N per sqr mm for the right. The moment at which the bolt starts to deform (this is the important part) is easy to calculate: Take the number left of the dot and multiply it by 10 times the number right of the dot:

8x80=640N for the left one and

6x80=480N for the right one - only 75% of the other


The higher both numbers the more the bolt can take before quitting service. This means that you should NEVER replace a bolt by a lower rated. But doing the opposite may also harm as a softer bolt will take bolt will deform while a hard one will break. Such bolts are often found in safety relevant parts like steering, brakes or suspension.A word on stailess:Stainless steel is in strenght about 12% under an 8.8 bolt. So take great care if you replace any standard steel bolt for an stainless one. But stainless is more elastic, so it deforms better and will not snap as quickly.

A word on cold welding

Cold welding happens when 2 identical metals are forced together. Only a thin coat of different material, either a coating, gas or an oxide allows treads being torqued. Uncoated materials will weld quickly. This happens often on stainless bolts as they have only a very thin oxide coat. To prevent this use a thin coat of copper grease on all bolts.