Prime-Line 9081832 Medium Split Lock Washers, #6, Zinc Plated Steel, 100-Pack

Split lock washers can help to prevent nuts or bolts from loosening by adding a spring tension force along with split edges that are designed to dig into the nut and material being fastened. For additional loosening resistance, use a lock washer on the nut side as well as on the head side. Split lock washers may not be as effective on extremely hard metals as the edges at the split cannot dig in to the surface as well.