Bolt and Nut Identification (page 3 of 3)

GENERAL INFORMATION

03-13

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.

Self-locking nuts

Self-locking nuts, i.e. nylon insert or deferred thread 
nuts can be re-used providing resistance can be felt 
when the locking portion of the nut passes over the 
thread of the bolt or stud.

Where self-locking nuts have been removed, it is 
advisable to replace them with new ones of the same 
type.