Trucks and trailers on our roads today use both standard and metric threads; metric fasteners are common but not universal in late model applications. The engine and major chassis components use metric fasteners almost exclusively (diameter and pitch are measured in millimeters). Most fasteners used on a truck are hex head fasteners (usually nonflanged); most metric fasteners are also nonflanged. Hardened flat washers are used under the bolt head, between the clamped components and the hex nut, to distribute the load: this prevents localized stress. The washers are cadmium- or zinc-plated and have a hardness rating of 38 to 45 Rockwell “C” hardness (HRC). Some fasteners, often those smaller than 1/2-inch diameter, have integral flanges that fit against the clamped surfaces. These flanges eliminate the requirement for washers.
Recommended reading: miscellaneous parts