Bolt Information

The Difference Between A Bolt And A Set Screw:


The Bolt: The Bolt is characterised as with out a full thread (i.e the thread does not reach the head), this part is known as the Shank. Bolts are generally not available in the shortest lengths as there is no room for the shank, and typically you will receive a Set Screw in place of a Bolt. The Bolt allows for movement.

The Set Screw: The Set Screw is characterised as with a full thread (i.e the thread reaches the head). Used to prevent relative motion (no movement).


Metric and Imperial Bolt Markings:

Metric Bolts have numbers marked on their heads to distinguish their grade. The most common being 4.8, 5.8, 8.8, 10.9, but there are a lot more grades. The manufactures markings can also be found on the head. 

  hex-class_4.8   Class 4.8
 hex-class_5.8 Class 5.8
 hex-class_8.8 Class 8.8
 hex-class_10.9 Class 10.9

The numbers on the bolt represent the bolts Tensile Ultimate Strength (before the point) and the bolts Tensile Yield Strength (after the point) please see Tensile Strength for a further breakdown and calculation of this. 


Imperial Bolts have line marking on their head to distinguish their grade, the most common grades being Grade 2, Grade 5 and Grade 8 .There are more grades but are not as commonly used.

The bolts are marked on their head's with diagonal lines to show what bolt grade they are:

 bolt_marking_grade2 Grade 2 Imperial Bolt is a standard hardware grade steel. This is the most common grade of steel fastener and is the least expensive. Grade 2 bolts have no head markings but a manufacture's marking may be present.
 bolt_marking_grade5 Grade 5 Imperial Bolt's are hardened to increase strength. Grade 5 bolts have 3 evenly spaced lines on the head
 bolt_marking_grade8 Grade 8 Imperial Bolt have been hardened more than grade 5 bolt's, thus they are stronger and are used in more demanding applications. Grade 8 Bolts have 6 evenly spaced radial lines on the head