Screw threads

SCREW THREADS

Metric threads to ISO standards are used. Damaged nuts, bolts and screws must always be discarded.

NOTE: Cleaning damaged threads with a die or tap impairs the strength and closeness of fit of the threads and is not recommended.

CAUTION: Always ensure that replacement bolts are at least equal in strength to those replaced.

Castellated nuts must not be slackened to accept a split-pin, except in recommended cases when this forms part of an adjustment. Do not allow oil or grease to enter blind threaded holes. The hydraulic action on screwing in the bolt or stud could split the housing. Always tighten a nut or bolt to the recommended torque figure. Damaged or corroded threads can affect the torque reading. To check or re-tighten a bolt or screw to a specified torque figure, first slacken a quarter of a turn, then retighten to the correct torque figure. Oil thread lightly before tightening to ensure a free running thread, except in the case of threads treated with sealant / lubricant, and self-locking nuts.

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.