Which of the following, has a nylon or fibre ring insert placed in the upper part of the nut? 

Explanation:

Self-locking nut (Simmonds nut):

• This is a special nut with a nylon or fiber ring insert placed in the upper part of the nut.
• The internal diameter of the ring is smaller than the core diameter of the bolt thread.
• The nut while tightening cuts its own thread on the nylon insert.
• This provides a positive grip and prevents the nut from loosening due to vibration.

Sawn nut (Wiles nut):

• In this type of locking, a slot is cut halfway across the nut. A screw is fitted with a clearance hole on the top part and matching thread on the lower part of the nut. Tightening of the nut provides positive locking for the nut.

Castle nuts:

• These nuts have a special provision in the form of slots for fixing split pins for locking the nuts. Slotted nuts are hexagonal-shaped throughout. In the case of castle nuts, the top part of the nut is cylindrical in shape.