Bolt and Nut Identification

GENERAL INFORMATION

03-11

Bolt and Nut Identification

Bolt identification

An ISO metric bolt or screw made of steel and larger 
than 6 mm in diameter can be identified by either of 
the symbols ISO M or M embossed or indented on 
top of the bolt head.

In addition to marks identifying the manufacturer, the 
top of the bolt head is also marked with symbols 
indicating the strength grade, e.g. 8.8; 10.9; 12.9;  
14.9. As an alternative, some bolts and screws have 
the M and strength grade symbol stamped on the 
flats of the hexagon.

Encapsulated bolts and screws

Encapsulated bolts and screws have a micro-
encapsulated locking agent pre-applied to the 
thread. They are identified by a coloured section 
which extends 360

°

 around the thread. The locking 

agent is released and activated by the assembly 
process and is then chemically cured to provide the 
locking action.

Unless a specific repair procedure states otherwise, 
encapsulated bolts may be re-used providing the 
threads are undamaged and the following procedure 
is adopted:

l

Remove loose adhesive from the bolt and 
housing threads.

l

Ensure threads are clean and free of oil and 
grease.

l

Apply an approved locking agent.

An encapsulated bolt may be replaced with a bolt of 
equivalent specification provided it is treated with an 
approved locking agent.