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Unified National Special sizes are the uncommon sizes that are difficult to find information about. The UNC and UNF sizes are common but UNS is not so much. The first time I ran into UNS was working on older Harley Davidson motorcycles. They used a lot of UNS sizes. It forced folks to purchase their fasteners from the dealer network for the most part. I had to purchase UNS taps, dies, and heli-coils while performing restorations on these machines. Reproduction fasteners are available too but I like to reuse as much of the original stuff as I can on some restorations.