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The RS Pro zinc plated locking nuts have a nylon insert. The nuts have been designed to remain secure despite severe vibration. Having a nylon insert makes the nut better at sealing the bolt thread against leaking water, oil and other liquids. When you insert the mating bolt into the nylon insert, the thread from the bolt compresses the nylon.
Type = Nylon Insert
Thread Size = M6
Material = Steel
Finish = Zinc Plated