Product Description Nylon Insert Lock Nuts are also known as stop nuts. Lock nuts are used for fastening to a bolt when mechanically joining materials together. The bolt threads into the nylon material are located at the top of the nut. The purpose of the nylon insert is to prevent backing off or loosening from vibration. From the Manufacturer Nylon Insert Lock Nuts are also known as stop nuts. Lock nuts are used for fastening to a bolt when mechanically joining materials together. The bolt threads into the nylon material are located at the top of the nut. The purpose of the nylon insert is to prevent backing off or loosening from vibration.