How to Identify the Strength Grade of Nuts and Bolts
Except for the very lowest grades, metric nuts and bolts all carry identification markings which indicate their strength.
On the head of each bolt are two numbers separated by a decimal point, the full list includes ten grades from 3.6 to 14.9 but in plant and machinery only grades 8.8, 9.8, 10.9 and 12.9 are normally found. The bolt head should also have a code – normally two or three letters – indicating the manufacturer.
Usually the markings are on the top of the bolt head but sometimes they are on the side.
The first number gives the breaking strength of the steel in the bolt. In round terms it is in 10’s of kg per square millimeter of the bolt cross second.
For example, a grade 12.9 bolt will not snap until every square millimetre of the bolt cross section is carrying a load of at least 120 kg. Similarly a grade 8.8 bolt will not snap until it is carrying a load of 80kg per square millimetre.
If a bolt is progressively loaded it extends very slightly – like a spring does and it springs back to its original length if the load is released. However, there is a limiting load where the bolt no longer springs back all the way and instead remains permanently stretched, this is know as the yield point. The second number on the head of the bolt indicates how much of the breaking strength the bolt can stand before it starts to stretch.
For example, a grade 12.9 bolt will begin to stretch when the load on the bolt is 90% of the breaking load. Similarly a grade 9.8 bolt will begin to stretch at 80% of its breaking load.
The strength grade marking is more difficult to see on metric nuts, sometimes it is given as a number, sometimes as a pattern of a dot and a line where the position of the line indicates the grade like the hands of a clock.
Where the grade is given as a number it may be stamped on the top of the nut or on one of the flat sides.
Clock face grade marks are shown in the illustration (right). Put the dot at the twelve O-Clock positions and the line shows the grade.
‘Clock Face’ Grade Marking
The rule is that the grade of the nut should always be the same as the bolt grade or one grade higher so: a grade 8.8 bolt should be fitted with a grade 8 or grade 9 nut. A grade 12.9 bolt should be fitted with a grade 12 or grade 14 nut.
