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. Alternatively, some bolts and screws have the M and strength grade symbol
stamped on the flats of the hexagon.
Encapsulated ('Patched') bolts and screws
Encapsulated ('patched') bolts and screws have a thread locking agent applied to the threads during manufacture. Most
thread locking agents are colored, the band of color extending for 360 degrees around the thread. Some locking agents
however, are neutral in color and may not be so easily identified apart from a slightly darker area of thread where the
locking agent has been applied. The locking agent is released and activated by the tightening process and is then
chemically cured to provide the locking action.
Self-locking bolts and screws
Unless stated in a specific repair procedure, self-locking bolts and screws i.e. nylon patched or trilobular thread can be
reused provided that resistance is felt when the locking portion enters the female thread.
Nylon patched bolts and screws have a locking agent either applied to, or inserted in the threaded portion. They are
identified by the presence of a colored section of thread extending approximately 180 degrees around the thread or by a
colored plug inserted into the bolt.