Spring (split lock) washers in zinc plated (M4 to M20), 304 stainless steel (M5 to M20) and galvanised (M8 to M20). They add tension under the nut to help stop it working loose.
-
From $0.03Zinc Spring Washers -
From $0.08304 Stainless Spring Washers -
From $0.10Galv Spring Washers