And the bolts and screws, fastening parts connections. Among the six with nut type 1, the most widely used for surface, grade C nut is coarser, high precision of the machinery, equipment or structure, Class A and class B nut for surface smooth, compared to higher accuracy of the machinery, equipment or structure. 2 the thickness of the hex nuts, much thicker M used often often need to installation. Hex nuts, thin thickness M thinner, used to connect parts by the surface with limited space. Hex nuts classified: Inside hexagonal nut and hex nuts.