A thread that will not assemble is a scrap part on site. ASTM bolting calls for Unified threads to ASME B1.1: Class 2A on studs and bolts, 2B in nuts. Up to 1 in. the thread is coarse (UNC); from 1-1/8 in. it is the 8-thread series (8UN), unless the order says otherwise. GO and NOT GO gauges show at once whether a thread is in tolerance, and pitch diameter, the size that decides how a thread fits, can be measured directly over three wires. Class 2A sits a small allowance below basic size, which leaves room for a thin plating: a zinc-plated thread must still accept a basic-size GO gauge (ASTM F1941).
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model dataThread series and classes from ASTM A962 (referred to by A193, A194 and A320) and ASME B1.1; gauging practice from ASME B1.2 and B1.3. Three-wire formula PD = M − 3W + 0.86603P with best-size wires (0.57735 × pitch), without the lead-angle correction. Threads after plating from ASTM F1941. Stud drawn 6 in. long; nut drawn to ASME B18.2.2 heavy hex dimensions. Gauges, wires, micrometer and calipers are generic.
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