3 1/4''是螺牙直径, 即直 3 1/4''每英吋8个螺纹__美制标准8N螺纹,也有12N & 16N等,一般UNC & UNF是英美统一制螺纹, Unless otherwise specified these nuts are tapped UNC-2B in sizes 1" and under, and 8UN-2B over 1" diameter.UNC-2B & 8UN-2B 说明螺牙的公差标准
3 1/4-8 UN 2B 为American Standard 英制螺纹
3 1/4 Nominal Size(Major Dia.=3.25 inch; Pitch Dia.=3.1688)
8 Threads per InchUN Unified National Thread
2b Class