Steel Externally Threaded Taper Pin With Hex Nut, Plain Finish, Standard Tolerance, #4 Pin Size, 1/4"-28 Thread Size, 0.248" Large End Diameter, 0.232" Small End Diameter, 3/4" Length (Pack of 5): Amazon.com:
Recessed Nuts, Recessed Pins and Industrial Pins
Carbon Steel Heavy Hex Grade 2h HDG Lubricated Hexagon Pin-Lock Nut - China Nut and Pin-Lock Nut
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Hillman 5/16-in Zinc-Plated Standard (SAE) Castle Nut in the Specialty Nuts department at Lowes.com