Bolt and Nut Identification (page 2 of 2)

GENERAL INFORMATION

03-12

Self-locking bolts and screws

Self-locking bolts and screws, i.e. nylon patched or 
trilobular thread can be re-used providing resistance 
can be felt when the locking portion enters the female 
thread.

Nylon patched bolts and screws have a locking agent 
pre-applied to the threads. They are identified by the 
presence of a coloured section of thread which 
extends for up to 180

°

 around the thread.

Trilobular bolts (i.e. Powerlok) have a special thread 
form which creates a slight interference with the 
thread of the hole or nut into which it is screwed.

DO NOT  re-use self-locking fasteners in critical 
locations (e.g. engine bearings, flywheel). Always 
use the correct replacement self-locking nut, bolt or 
screw. 

DO NOT  fit non self-locking fasteners in applications 
where a self-locking nut, bolt or screw is specified.

Nut identification

A nut with an ISO metric thread is marked on one 
face or on one of the flats of the hexagon with the 
strength grade symbol 8, 12, or 14. Some nuts with a 
strength grade 4, 5 or 6 are also marked and some 
have the metric symbol M on the flat opposite the 
strength grade marking.

A clock face system is sometimes used as an 
alternative method of indicating the strength grade. 
The external chamfers or a face of the nut is marked 
in a position relative to the appropriate hour mark on 
a clock face to indicate the strength grade.

A dot is used to locate the 12 o'clock position and a 
dash to indicate the strength grade. If the grade is 
above 12, two dots identify the 12 o'clock position.

When tightening a slotted or castellated nut, never 
loosen it to insert a split pin except where 
recommended as part of an adjustment. If difficulty is 
experienced, alternative washers or nuts should be 
selected, or the washer thickness reduced.

Where bearing pre-load is involved, nuts should be 
tightened in accordance with special instructions.