This is a set of 3BA (British Association) shakeproof washers with external teeth, made from black steel. 3BA 0.167 in (4.24 mm) 0.313 in (7.95 mm) 0.026 in (0.68 mm). Shakeproof washers have serrated teeth that lock surfaces together when tightened, and resist coming loose under vibration.
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