Here is a trick you can use to help hold a guitar pot in place when a lock washer can’t be fit in. You can bend the flat washer to give it a slight V shape. This V shape will exert pressure on the Hex nut and help the Pot from getting loose over time.
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Shanghai Jian & Mei Industry and Trade Co., Ltd. is the company which manufacture and export fasteners and metal hardware, founded in 2006, with import-export authority. Now, 80% products export to all over the world and 20% sell in domestic market.